linux-kernel - Re: [PATCH] kvm/svm: PKU not currently supported

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Date:   Thu, 19 Dec 2019 13:49:48 -0600
From:   John Allen <john.allen@....com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, rkrcmar@...hat.com
Subject: Re: [PATCH] kvm/svm: PKU not currently supported

On Thu, Dec 19, 2019 at 08:09:57PM +0100, Vitaly Kuznetsov wrote:
> John Allen <john.allen@....com> writes:
> 
> > Current SVM implementation does not have support for handling PKU. Guests
> > running on a host with future AMD cpus that support the feature will read
> > garbage from the PKRU register and will hit segmentation faults on boot as
> > memory is getting marked as protected that should not be. Ensure that cpuid
> > from SVM does not advertise the feature.
> >
> > Signed-off-by: John Allen <john.allen@....com>
> > ---
> >  arch/x86/kvm/svm.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index 122d4ce3b1ab..f911aa1b41c8 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -5933,6 +5933,8 @@ static void svm_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2 *entry)
> >  		if (avic)
> >  			entry->ecx &= ~bit(X86_FEATURE_X2APIC);
> >  		break;
> > +	case 0x7:
> > +		entry->ecx &= ~bit(X86_FEATURE_PKU);
> 
> Would it make more sense to introduce kvm_x86_ops->pku_supported() (and
> return false for SVM and boot_cpu_has(X86_FEATURE_PKU) for vmx) so we
> don't set the bit in the first place?

Yes, I think you're right. I had initially planned to do it that way so I
already have a patch ready. I'll send it up pronto.

> 
> >  	case 0x80000001:
> >  		if (nested)
> >  			entry->ecx |= (1 << 2); /* Set SVM bit */
> 
> -- 
> Vitaly
>