linux-kernel - Re: [PATCH 05/12] KVM: x86: Use KVM-governed feature framework to track "XSAVES enabled"

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Message-ID: <Y/ZlRDuRivMDKGwo@google.com>
Date:   Wed, 22 Feb 2023 10:56:04 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/12] KVM: x86: Use KVM-governed feature framework to
 track "XSAVES enabled"

On Tue, Feb 21, 2023, Yu Zhang wrote:
> On Fri, Feb 17, 2023 at 03:10:15PM -0800, Sean Christopherson wrote:
> > Use the governed feature framework to track if XSAVES is "enabled", i.e.
> > if XSAVES can be used by the guest.  Add a comment in the SVM code to
> > explain the very unintuitive logic of deliberately NOT checking if XSAVES
> > is enumerated in the guest CPUID model.
> > 
> > No functional change intended.
> 
> xsaves_enabled in struct kvm_vcpu_arch is no longer used. But instead of
> just deleting it, maybe we could move 'bool load_eoi_exitmap_pending' to
> its place, so 7 bytes can be saved for each struct kvm_vcpu_arch:

I prefer leaving load_eoi_exitmap_pending where it is so that it's co-located with
ioapic_handled_vectors.  I agree wasting 7 bytes is unfortunate, but I don't want
to take an ad hoc approach to shrinking per-vCPU structs.  See the link below for
more discussion.

https://lore.kernel.org/all/20230213163351.30704-1-minipli@grsecurity.net

> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index cd660de02f7b..0eef5469c165 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -740,7 +740,6 @@ struct kvm_vcpu_arch {
>         u64 efer;
>         u64 apic_base;
>         struct kvm_lapic *apic;    /* kernel irqchip context */
> -       bool load_eoi_exitmap_pending;
>         DECLARE_BITMAP(ioapic_handled_vectors, 256);
>         unsigned long apic_attention;
>         int32_t apic_arb_prio;
> @@ -750,7 +749,7 @@ struct kvm_vcpu_arch {
>         u64 smi_count;
>         bool at_instruction_boundary;
>         bool tpr_access_reporting;
> -       bool xsaves_enabled;
> +       bool load_eoi_exitmap_pending;
>         bool xfd_no_write_intercept;
>         u64 ia32_xss;
>         u64 microcode_version;
> 
> B.R.
> Yu
>