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Date: Mon, 30 Oct 2023 13:17:21 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Jonathan Corbet <corbet@....net>,
Wanpeng Li <wanpengli@...cent.com>, x86@...nel.org,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86/kvm/async_pf: Use separate percpu variable to
track the enabling of asyncpf
On 10/25/2023 5:10 PM, Vitaly Kuznetsov wrote:
> Xiaoyao Li <xiaoyao.li@...el.com> 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@google.com/T/#u
>>
>> Suggested-by: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>> ---
>> 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());
>> }
>
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