Re: [PATCH v8 04/27] x86/fpu/xstate: Introduce XSAVES system states

From: Yu-cheng Yu
Date: Fri Aug 16 2019 - 16:09:44 EST

On Fri, 2019-08-16 at 21:56 +0200, Thomas Gleixner wrote:
> On Tue, 13 Aug 2019, Yu-cheng Yu wrote:
> > +/*
> > + * On context switches, XSAVE states are not restored until returning
> > + * to user-mode. FPU registers need to be restored before any changes,
> > + * and protected by fpregs_lock()/fpregs_unlock().
>
> I really had to read this comment twice to figure out what it means.
>
> > + */
> > +static inline void modify_fpu_regs_begin(void)
>
> Please use a proper name space. fpu_regs_....
>
> > +{
> > + fpregs_lock();
> > + if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > + __fpregs_load_activate();
> > +}
> > +
> > +static inline void modify_fpu_regs_end(void)
> > +{
> > + fpregs_unlock();
> > +}
>
> Also why are those inlines in this particular patch? I see no relation at all.
>
> > /*
> > * MXCSR and XCR definitions:
> > */
> > diff --git a/arch/x86/include/asm/fpu/xstate.h
> > b/arch/x86/include/asm/fpu/xstate.h
> > index 9ded9532257d..970bbd303cfb 100644
> > --- a/arch/x86/include/asm/fpu/xstate.h
> > +++ b/arch/x86/include/asm/fpu/xstate.h
> > @@ -21,9 +21,6 @@
> > #define XSAVE_YMM_SIZE 256
> > #define XSAVE_YMM_OFFSET (XSAVE_HDR_SIZE + XSAVE_HDR_OFFSET)
> >
> > -/* Supervisor features */
> > -#define XFEATURE_MASK_SUPERVISOR (XFEATURE_MASK_PT)
> > -
> > /* All currently supported features */
> > #define SUPPORTED_XFEATURES_MASK (XFEATURE_MASK_FP | \
> > XFEATURE_MASK_SSE | \
> > @@ -42,6 +39,7 @@
> > #endif
> >
> > extern u64 xfeatures_mask_user;
> > +extern u64 xfeatures_mask_all;
> > extern u64 xstate_fx_sw_bytes[USER_XSTATE_FX_SW_WORDS];
> >
> > extern void __init update_regset_xstate_info(unsigned int size,
> > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> > index 12c70840980e..31d3cd70b5df 100644
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -294,12 +294,16 @@ void fpu__drop(struct fpu *fpu)
> > * Clear FPU registers by setting them up from
> > * the init fpstate:
> > */
> > -static inline void copy_init_fpstate_to_fpregs(void)
> > +static inline void copy_init_fpstate_to_fpregs(u64 features_mask)
> > {
> > fpregs_lock();
> >
> > + /*
> > + * Only XSAVES user states are copied.
> > + * System states are preserved.
>
> Fits nicely in one line and aside of that this comment is blatantly
> wrong. See that caller:
>
> > + copy_init_fpstate_to_fpregs(xfeatures_mask_all);
>
> xfeatures_mask_all includes xfeatures_mask_system unless I'm missing
> something.
>
> > + */
> > if (use_xsave())
> > - copy_kernel_to_xregs(&init_fpstate.xsave, -1);
> > + copy_kernel_to_xregs(&init_fpstate.xsave, features_mask);
> > else if (static_cpu_has(X86_FEATURE_FXSR))
> > copy_kernel_to_fxregs(&init_fpstate.fxsave);
>
> The change of this function should also be split out into a separate
> patch. This one is way too big to be reviewable.
>
> > else
> > @@ -318,7 +322,21 @@ static inline void copy_init_fpstate_to_fpregs(void)
> > * Called by sys_execve(), by the signal handler code and by various
> > * error paths.
> > */
> > -void fpu__clear(struct fpu *fpu)
> > +void fpu__clear_user_states(struct fpu *fpu)
> > +{
> > + WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an
> > anomaly */
>
> 1) Please do not use tail comments. They break the reading flow.
>
> 2) Please do not comment the obvious. Put comments where they make sense. I
> know you copied it, but that does not make it any better.
>
> > + fpu__drop(fpu);
> > +
> > + /*
> > + * Make sure fpstate is cleared and initialized.
> > + */
> > + fpu__initialize(fpu);
> > + if (static_cpu_has(X86_FEATURE_FPU))
> > + copy_init_fpstate_to_fpregs(xfeatures_mask_user);
> > +}
> > +
> > +void fpu__clear_all(struct fpu *fpu)
> > {
> > WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an
> > anomaly */
> > @@ -329,7 +347,7 @@ void fpu__clear(struct fpu *fpu)
> > */
> > fpu__initialize(fpu);
> > if (static_cpu_has(X86_FEATURE_FPU))
> > - copy_init_fpstate_to_fpregs();
> > + copy_init_fpstate_to_fpregs(xfeatures_mask_all);
> > }
> >
> > /*
> > diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
> > index 73fed33e5bda..0a0ba584a533 100644
> > --- a/arch/x86/kernel/fpu/init.c
> > +++ b/arch/x86/kernel/fpu/init.c
> > @@ -217,16 +217,6 @@ static void __init
> > fpu__init_system_xstate_size_legacy(void)
> > fpu_user_xstate_size = fpu_kernel_xstate_size;
> > }
> >
> > -/*
> > - * Find supported xfeatures based on cpu features and command-line input.
> > - * This must be called after fpu__init_parse_early_param() is called and
> > - * xfeatures_mask is enumerated.
> > - */
> > -u64 __init fpu__get_supported_xfeatures_mask(void)
> > -{
> > - return SUPPORTED_XFEATURES_MASK;
> > -}
> > -
> > /* Legacy code to initialize eager fpu mode. */
> > static void __init fpu__init_system_ctx_switch(void)
> > {
> > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> > index 8a63f07cf400..4ecf1764a971 100644
> > --- a/arch/x86/kernel/fpu/signal.c
> > +++ b/arch/x86/kernel/fpu/signal.c
> > @@ -285,7 +285,7 @@ static int __fpu__restore_sig(void __user *buf, void
> > __user *buf_fx, int size)
> > IS_ENABLED(CONFIG_IA32_EMULATION));
> >
> > if (!buf) {
> > - fpu__clear(fpu);
> > + fpu__clear_user_states(fpu);
> > return 0;
> > }
> >
> > @@ -407,7 +407,7 @@ static int __fpu__restore_sig(void __user *buf, void
> > __user *buf_fx, int size)
> >
> > err_out:
> > if (ret)
> > - fpu__clear(fpu);
> > + fpu__clear_user_states(fpu);
> > return ret;
> > }
> >
> > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> > index d560e8861a3c..9fbe73c546df 100644
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -61,9 +61,19 @@ static short xsave_cpuid_features[] __initdata = {
> > */
> > u64 xfeatures_mask_user __read_mostly;
> >
> > +/*
> > + * Supported XSAVES system states.
> > + */
> > +static u64 xfeatures_mask_system __read_mostly;
> > +
> > +/*
> > + * Combined XSAVES system and user states.
> > + */
> > +u64 xfeatures_mask_all __read_mostly;
> > +
> > static unsigned int xstate_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX -
> > 1] = -1};
> > static unsigned int xstate_sizes[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX -
> > 1] = -1};
> > -static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask_user)*8];
> > +static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask_all)*8];
>
> [sizeof(...) * 8]
>
> >
> > /*
> > * The XSAVE area of kernel can be in standard or compacted format;
> > @@ -79,7 +89,7 @@ unsigned int fpu_user_xstate_size;
> > */
> > int cpu_has_xfeatures(u64 xfeatures_needed, const char **feature_name)
> > {
> > - u64 xfeatures_missing = xfeatures_needed & ~xfeatures_mask_user;
> > + u64 xfeatures_missing = xfeatures_needed & ~xfeatures_mask_all;
> >
> > if (unlikely(feature_name)) {
> > long xfeature_idx, max_idx;
> > @@ -158,7 +168,7 @@ void fpstate_sanitize_xstate(struct fpu *fpu)
> > * None of the feature bits are in init state. So nothing else
> > * to do for us, as the memory layout is up to date.
> > */
> > - if ((xfeatures & xfeatures_mask_user) == xfeatures_mask_user)
> > + if ((xfeatures & xfeatures_mask_all) == xfeatures_mask_all)
> > return;
> >
> > /*
> > @@ -213,28 +223,27 @@ void fpstate_sanitize_xstate(struct fpu *fpu)
> > */
> > void fpu__init_cpu_xstate(void)
> > {
> > - if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask_user)
> > + if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask_all)
> > return;
> > /*
> > * XCR_XFEATURE_ENABLED_MASK sets the features that are managed
> > * by XSAVE{C, OPT} and XRSTOR. Only XSAVE user states can be
> > * set here.
> > */
> > -
> > - xfeatures_mask_user &= ~XFEATURE_MASK_SUPERVISOR;
> > -
> > cr4_set_bits(X86_CR4_OSXSAVE);
> > xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask_user);
> > +
> > + /*
> > + * MSR_IA32_XSS controls which system (not user) states are
>
> We know that system state is not including user state. Please stop
> documenting the obvious.
>
> > + * to be managed by XSAVES.
> > + */
> > + if (boot_cpu_has(X86_FEATURE_XSAVES))
> > + wrmsrl(MSR_IA32_XSS, xfeatures_mask_system);
> > }
> >
> > -/*
> > - * Note that in the future we will likely need a pair of
> > - * functions here: one for user xstates and the other for
> > - * system xstates. For now, they are the same.
> > - */
> > static int xfeature_enabled(enum xfeature xfeature)
> > {
> > - return !!(xfeatures_mask_user & BIT_ULL(xfeature));
> > + return !!(xfeatures_mask_all & BIT_ULL(xfeature));
> > }
> >
> > /*
> > @@ -340,7 +349,7 @@ static int xfeature_is_aligned(int xfeature_nr)
> > */
> > static void __init setup_xstate_comp(void)
> > {
> > - unsigned int xstate_comp_sizes[sizeof(xfeatures_mask_user)*8];
> > + unsigned int xstate_comp_sizes[sizeof(xfeatures_mask_all)*8];
>
> See above
>
> > int i;
> >
> > /*
> > @@ -413,7 +422,7 @@ static void __init setup_init_fpu_buf(void)
> > print_xstate_features();
> >
> > if (boot_cpu_has(X86_FEATURE_XSAVES))
> > - init_fpstate.xsave.header.xcomp_bv = BIT_ULL(63) |
> > xfeatures_mask_user;
> > + init_fpstate.xsave.header.xcomp_bv = BIT_ULL(63) |
> > xfeatures_mask_all;
> >
> > /*
> > * Init all the features state with header.xfeatures being 0x0
> > @@ -436,7 +445,7 @@ static int xfeature_uncompacted_offset(int xfeature_nr)
> > * format. Checking a system state's uncompacted offset is
> > * an error.
> > */
> > - if (XFEATURE_MASK_SUPERVISOR & BIT_ULL(xfeature_nr)) {
> > + if (~xfeatures_mask_user & BIT_ULL(xfeature_nr)) {
>
> Sigh. Why can't this use xfeatures_mask_system? That would be too obvious.
>
> > WARN_ONCE(1, "No fixed offset for xstate %d\n",
> > xfeature_nr);
> > return -1;
> > }
> > @@ -608,15 +617,12 @@ static void do_extra_xstate_size_checks(void)
> >
> >
> > /*
> > - * Get total size of enabled xstates in XCR0/xfeatures_mask_user.
> > + * Get total size of enabled xstates in XCR0 | IA32_XSS.
> > *
> > * Note the SDM's wording here. "sub-function 0" only enumerates
> > * the size of the *user* states. If we use it to size a buffer
> > * that we use 'XSAVES' on, we could potentially overflow the
> > * buffer because 'XSAVES' saves system states too.
> > - *
> > - * Note that we do not currently set any bits on IA32_XSS so
> > - * 'XCR0 | IA32_XSS == XCR0' for now.
> > */
> > static unsigned int __init get_xsaves_size(void)
> > {
> > @@ -698,6 +704,7 @@ static int __init init_xstate_size(void)
> > */
> > static void fpu__init_disable_system_xstate(void)
> > {
> > + xfeatures_mask_all = 0;
> > xfeatures_mask_user = 0;
> > cr4_clear_bits(X86_CR4_OSXSAVE);
> > setup_clear_cpu_cap(X86_FEATURE_XSAVE);
> > @@ -733,10 +740,23 @@ void __init fpu__init_system_xstate(void)
> > return;
> > }
> >
> > + /*
> > + * Find user states supported by the processor.
> > + * Only these bits can be set in XCR0.
> > + */
> > cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
> > xfeatures_mask_user = eax + ((u64)edx << 32);
> >
> > - if ((xfeatures_mask_user & XFEATURE_MASK_FPSSE) !=
> > XFEATURE_MASK_FPSSE) {
> > + /*
> > + * Find system states supported by the processor.
> > + * Only these bits can be set in IA32_XSS MSR.
> > + */
> > + cpuid_count(XSTATE_CPUID, 1, &eax, &ebx, &ecx, &edx);
> > + xfeatures_mask_system = ecx + ((u64)edx << 32);
> > +
> > + xfeatures_mask_all = xfeatures_mask_user | xfeatures_mask_system;
> > +
> > + if ((xfeatures_mask_all & XFEATURE_MASK_FPSSE) !=
> > XFEATURE_MASK_FPSSE) {
>
> xfeatures_mask_all is wrong here. FPSSE is clearly user state.
>
> > /*
> > * This indicates that something really unexpected happened
> > * with the enumeration. Disable XSAVE and try to continue
> > @@ -751,10 +771,12 @@ void __init fpu__init_system_xstate(void)
> > */
> > for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
> > if (!boot_cpu_has(xsave_cpuid_features[i]))
> > - xfeatures_mask_user &= ~BIT_ULL(i);
> > + xfeatures_mask_all &= ~BIT_ULL(i);
> > }
> >
> > - xfeatures_mask_user &= fpu__get_supported_xfeatures_mask();
> > + xfeatures_mask_all &= SUPPORTED_XFEATURES_MASK;
> > + xfeatures_mask_user &= xfeatures_mask_all;
> > + xfeatures_mask_system &= xfeatures_mask_all;
> >
> > /* Enable xstate instructions to be able to continue with
> > initialization: */
> > fpu__init_cpu_xstate();
> > @@ -766,7 +788,7 @@ void __init fpu__init_system_xstate(void)
> > * Update info used for ptrace frames; use standard-format size and
> > no
> > * system xstates:
> > */
> > - update_regset_xstate_info(fpu_user_xstate_size, xfeatures_mask_user
> > & ~XFEATURE_MASK_SUPERVISOR);
> > + update_regset_xstate_info(fpu_user_xstate_size,
> > xfeatures_mask_user);
> >
>
> And exactly this hunk shows that the whole refactoring approach is wrong
> from the very beginning. I stared at that in the previous patch already and
> had the feeling that it's bogus.
>
> Just doing a s/xfeatures_mask/xfeatures_mask_user/g really does not make
> any sense. Simply because the current code assumes that xfeatures_mask ==
> xfeatures_mask_all. So if a global rename is the right approach then
> s/xfeatures_mask/xfeatures_mask_all/ and not that completely backwards
> rename to _user.
>
> That refactoring wants to be done in the following steps:
>
> 1) Introduce xfeatures_mask_user and initialize it with
>
> xfeatures_mask_user = xfeatures_mask ^ ~XFEATURE_MASK_SUPERVISOR;
>
> 2) Fix up the usage sites in reviewable chunks. It does not matter
> whether that could be folded into a larger all in one patch. What
> matters is that it makes sense and is reviewable.
>
> 3) Change the signature of copy_init_fpstate_to_fpregs() so it takes a
> mask and fix up the call sites accordingly. Without the bogus comment
> of course.
>
> 4) Introduce xfeatures_mask_system and eventually needed helper functions.
>
> 5) Change the affected usage sites
>
> Details may be slightly different but you get the idea.

I will work on it. Thanks!

Yu-cheng