Re: [PATCH v2] x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM

[Brian Gerst]

On Fri, Aug 21, 2020 at 6:56 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>
> Don't use RDPID in the paranoid entry flow, as it can consume a KVM
> guest's MSR_TSC_AUX value if an NMI arrives during KVM's run loop.
>
> In general, the kernel does not need TSC_AUX because it can just use
> __this_cpu_read(cpu_number) to read the current processor id.  It can
> also just block preemption and thread migration at its will, therefore
> it has no need for the atomic rdtsc+vgetcpu provided by RDTSCP.  For this
> reason, as a performance optimization, KVM loads the guest's TSC_AUX when
> a CPU first enters its run loop.  On AMD's SVM, it doesn't restore the
> host's value until the CPU exits the run loop; VMX is even more aggressive
> and defers restoring the host's value until the CPU returns to userspace.
>
> This optimization obviously relies on the kernel not consuming TSC_AUX,
> which falls apart if an NMI arrives during the run loop and uses RDPID.
> Removing it would be painful, as both SVM and VMX would need to context
> switch the MSR on every VM-Enter (for a cost of 2x WRMSR), whereas using
> LSL instead RDPID is a minor blip.
>
> Both SAVE_AND_SET_GSBASE and GET_PERCPU_BASE are only used in paranoid entry,
> therefore the patch can just remove the RDPID alternative.
>
> Fixes: eaad981291ee3 ("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Chang Seok Bae <chang.seok.bae@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Sasha Levin <sashal@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Reported-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Debugged-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/entry/calling.h | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 98e4d8886f11..ae9b0d4615b3 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -374,12 +374,14 @@ For 32-bit we have the following conventions - kernel is built with
>   * Fetch the per-CPU GSBASE value for this processor and put it in @reg.
>   * We normally use %gs for accessing per-CPU data, but we are setting up
>   * %gs here and obviously can not use %gs itself to access per-CPU data.
> + *
> + * Do not use RDPID, because KVM loads guest's TSC_AUX on vm-entry and
> + * may not restore the host's value until the CPU returns to userspace.
> + * Thus the kernel would consume a guest's TSC_AUX if an NMI arrives
> + * while running KVM's run loop.
>   */
>  .macro GET_PERCPU_BASE reg:req
> -       ALTERNATIVE \
> -               "LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
> -               "RDPID  \reg", \
> -               X86_FEATURE_RDPID
> +       LOAD_CPU_AND_NODE_SEG_LIMIT \reg
>         andq    $VDSO_CPUNODE_MASK, \reg
>         movq    __per_cpu_offset(, \reg, 8), \reg
>  .endm

LOAD_CPU_AND_NODE_SEG_LIMIT can be merged into this, as its only
purpose was to work around using CPP macros in an alternative.

--
Brian Gerst