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Message-ID: <87a5s73w53.fsf@redhat.com>
Date: Wed, 25 Oct 2023 11:10:32 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Xiaoyao Li <xiaoyao.li@...el.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,
Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v2 1/2] x86/kvm/async_pf: Use separate percpu variable
to track the enabling of asyncpf
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.
>
> 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.
>
> pr_debug("disable async PF for cpu %d\n", smp_processor_id());
> }
--
Vitaly
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