Re: [PATCH v2 1/2] x86/kvm/async_pf: Use separate percpu variable to track the enabling of asyncpf

[Xiaoyao Li]

On 10/25/2023 5:10 PM, Vitaly Kuznetsov wrote:
> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
>
> > Refer to commit fd10cde9294f ("KVM paravirt: Add async PF initialization
> > to PV guest") and commit 344d9588a9df ("KVM: Add PV MSR to enable
> > asynchronous page faults delivery"). It turns out that at the time when
> > asyncpf was introduced, the purpose was defining the shared PV data 'struct
> > kvm_vcpu_pv_apf_data' with the size of 64 bytes. However, it made a mistake
> > and defined the size to 68 bytes, which failed to make fit in a cache line
> > and made the code inconsistent with the documentation.
>
> Oh, I actually though it was done on purpose :-) 'enabled' is not
> accessed by the host, it's only purpose is to cache MSR_KVM_ASYNC_PF_EN.

I didn't find any clue to show it was on purpose, so thought it was a 
mistake. Anyway, if the fact is it was done on purpose and people now 
still accept it. I can drop this patch, and write another one to 
document it's intentional instead.

> > Below justification quoted from Sean[*]
> >
> >    KVM (the host side) has *never* read kvm_vcpu_pv_apf_data.enabled, and
> >    the documentation clearly states that enabling is based solely on the
> >    bit in the synthetic MSR.
> >
> >    So rather than update the documentation, fix the goof by removing the
> >    enabled filed and use the separate percpu variable instread.
> >    KVM-as-a-host obviously doesn't enforce anything or consume the size,
> >    and changing the header will only affect guests that are rebuilt against
> >    the new header, so there's no chance of ABI breakage between KVM and its
> >    guests. The only possible breakage is if some other hypervisor is
> >    emulating KVM's async #PF (LOL) and relies on the guest to set
> >    kvm_vcpu_pv_apf_data.enabled. But (a) I highly doubt such a hypervisor
> >    exists, (b) that would arguably be a violation of KVM's "spec", and
> >    (c) the worst case scenario is that the guest would simply lose async
> >    #PF functionality.
> >
> > [*] https://lore.kernel.org/all/ZS7ERnnRqs8Fl0ZF@xxxxxxxxxx/T/#u
> >
> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > ---
> >   Documentation/virt/kvm/x86/msr.rst   |  1 -
> >   arch/x86/include/uapi/asm/kvm_para.h |  1 -
> >   arch/x86/kernel/kvm.c                | 11 ++++++-----
> >   3 files changed, 6 insertions(+), 7 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/x86/msr.rst b/Documentation/virt/kvm/x86/msr.rst
> > index 9315fc385fb0..f6d70f99a1a7 100644
> > --- a/Documentation/virt/kvm/x86/msr.rst
> > +++ b/Documentation/virt/kvm/x86/msr.rst
> > @@ -204,7 +204,6 @@ data:
> >   		__u32 token;
> >   
> >   		__u8 pad[56];
> > -		__u32 enabled;
> >   	  };
> >   
> >   	Bits 5-4 of the MSR are reserved and should be zero. Bit 0 is set to 1
> > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> > index 6e64b27b2c1e..605899594ebb 100644
> > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > @@ -142,7 +142,6 @@ struct kvm_vcpu_pv_apf_data {
> >   	__u32 token;
> >   
> >   	__u8 pad[56];
> > -	__u32 enabled;
> >   };
> >   
> >   #define KVM_PV_EOI_BIT 0
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index b8ab9ee5896c..388a3fdd3cad 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -65,6 +65,7 @@ static int __init parse_no_stealacc(char *arg)
> >   
> >   early_param("no-steal-acc", parse_no_stealacc);
> >   
> > +static DEFINE_PER_CPU_READ_MOSTLY(bool, async_pf_enabled);
>
> Would it make a difference is we replace this with a cpumask? I realize
> that we need to access it on all CPUs from hotpaths but this mask will
> rarely change so maybe there's no real perfomance hit?
>
> >   static DEFINE_PER_CPU_DECRYPTED(struct kvm_vcpu_pv_apf_data, apf_reason) __aligned(64);
> >   DEFINE_PER_CPU_DECRYPTED(struct kvm_steal_time, steal_time) __aligned(64) __visible;
> >   static int has_steal_clock = 0;
> > @@ -244,7 +245,7 @@ noinstr u32 kvm_read_and_reset_apf_flags(void)
> >   {
> >   	u32 flags = 0;
> >   
> > -	if (__this_cpu_read(apf_reason.enabled)) {
> > +	if (__this_cpu_read(async_pf_enabled)) {
> >   		flags = __this_cpu_read(apf_reason.flags);
> >   		__this_cpu_write(apf_reason.flags, 0);
> >   	}
> > @@ -295,7 +296,7 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_kvm_asyncpf_interrupt)
> >   
> >   	inc_irq_stat(irq_hv_callback_count);
> >   
> > -	if (__this_cpu_read(apf_reason.enabled)) {
> > +	if (__this_cpu_read(async_pf_enabled)) {
> >   		token = __this_cpu_read(apf_reason.token);
> >   		kvm_async_pf_task_wake(token);
> >   		__this_cpu_write(apf_reason.token, 0);
> > @@ -362,7 +363,7 @@ static void kvm_guest_cpu_init(void)
> >   		wrmsrl(MSR_KVM_ASYNC_PF_INT, HYPERVISOR_CALLBACK_VECTOR);
> >   
> >   		wrmsrl(MSR_KVM_ASYNC_PF_EN, pa);
> > -		__this_cpu_write(apf_reason.enabled, 1);
> > +		__this_cpu_write(async_pf_enabled, 1);
>
> As 'async_pf_enabled' is bool, it would probably be more natural to
> write
>
> 	__this_cpu_write(async_pf_enabled, true);
>
> >   		pr_debug("setup async PF for cpu %d\n", smp_processor_id());
> >   	}
> >   
> > @@ -383,11 +384,11 @@ static void kvm_guest_cpu_init(void)
> >   
> >   static void kvm_pv_disable_apf(void)
> >   {
> > -	if (!__this_cpu_read(apf_reason.enabled))
> > +	if (!__this_cpu_read(async_pf_enabled))
> >   		return;
> >   
> >   	wrmsrl(MSR_KVM_ASYNC_PF_EN, 0);
> > -	__this_cpu_write(apf_reason.enabled, 0);
> > +	__this_cpu_write(async_pf_enabled, 0);
>
> ... and 'false' here.

sure, I can do it in a v3, if v3 is needed.

> >   
> >   	pr_debug("disable async PF for cpu %d\n", smp_processor_id());
> >   }