linux-kernel - Re: [PATCH v7 5/7] kvm: x86: Add CET CR4 bit and XSS support

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Date:   Fri, 18 Oct 2019 09:58:02 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Yang Weijiang <weijiang.yang@...el.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v7 5/7] kvm: x86: Add CET CR4 bit and XSS support

On Thu, Oct 17, 2019 at 12:56:42PM -0700, Sean Christopherson wrote:
> On Wed, Oct 02, 2019 at 12:05:23PM -0700, Jim Mattson wrote:
> > On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@...el.com> wrote:
> > >
> > > CR4.CET(bit 23) is master enable bit for CET feature.
> > > Previously, KVM did not support setting any bits in XSS
> > > so it's hardcoded to check and inject a #GP if Guest
> > > attempted to write a non-zero value to XSS, now it supports
> > > CET related bits setting.
> > >
> > > Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> > > Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> > > Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> > > ---
> > >  arch/x86/include/asm/kvm_host.h |  4 +++-
> > >  arch/x86/kvm/cpuid.c            | 11 +++++++++--
> > >  arch/x86/kvm/vmx/vmx.c          |  6 +-----
> > >  3 files changed, 13 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > index d018df8c5f32..8f97269d6d9f 100644
> > > --- a/arch/x86/include/asm/kvm_host.h
> > > +++ b/arch/x86/include/asm/kvm_host.h
> > > @@ -90,7 +90,8 @@
> > >                           | X86_CR4_PGE | X86_CR4_PCE | X86_CR4_OSFXSR | X86_CR4_PCIDE \
> > >                           | X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
> > >                           | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
> > > -                         | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP))
> > > +                         | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
> > > +                         | X86_CR4_CET))
> > >
> > >  #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
> > >
> > > @@ -623,6 +624,7 @@ struct kvm_vcpu_arch {
> > >
> > >         u64 xcr0;
> > >         u64 guest_supported_xcr0;
> > > +       u64 guest_supported_xss;
> > >         u32 guest_xstate_size;
> > >
> > >         struct kvm_pio_request pio;
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index 0a47b9e565be..dd3ddc6daa58 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -120,8 +120,15 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> > >         }
> > >
> > >         best = kvm_find_cpuid_entry(vcpu, 0xD, 1);
> > > -       if (best && (best->eax & (F(XSAVES) | F(XSAVEC))))
> > > -               best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> > > +       if (best && (best->eax & (F(XSAVES) | F(XSAVEC)))) {
> > 
> > Is XSAVEC alone sufficient? Don't we explicitly need XSAVES to
> > save/restore the extended state components enumerated by IA32_XSS?
> 
> Hmm, I think the check would be ok as-is if vcpu->arch.ia32_xss is used
> below, as ia32_xss is guaranteed to be zero if XSAVES isn't supported.
> 
Thanks Sean having me re-capture this reply thread, it's lost in my
folder.
I added kvm_x86_ops->xsaves_supported() in kvm_supported_xss() and it
returns 0 if xsaves is not supported which suggested by Jim.

> > > +               u64 kvm_xss = kvm_supported_xss();
> > > +
> > > +               best->ebx =
> > > +                       xstate_required_size(vcpu->arch.xcr0 | kvm_xss, true);
> > 
> > Shouldn't this size be based on the *current* IA32_XSS value, rather
> > than the supported IA32_XSS bits? (i.e.
> > s/kvm_xss/vcpu->arch.ia32_xss/)
> 
> Ya.
>
I'm not sure if I understand correctly, kvm_xss is what KVM supports,
but arch.ia32_xss reflects what guest currently is using, shoudn't CPUID
report what KVM supports instead of current status?
Will CPUID match current IA32_XSS status if guest changes it runtime?

> > > +               vcpu->arch.guest_supported_xss = best->ecx & kvm_xss;
> > 
> > Shouldn't unsupported bits in best->ecx be masked off, so that the
> > guest CPUID doesn't mis-report the capabilities of the vCPU?
> 
> I thought KVM liked to let userspace blow off their foot whenever possible?
> KVM already enumerated what features are supported, it's a userspace bug
> if it ignores the enumeration.
> 
> > > +       } else {
> > > +               vcpu->arch.guest_supported_xss = 0;
> > > +       }
> > >
> > >         /*