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Message-ID: <ZGfius5UkckpUyXl@google.com>
Date: Fri, 19 May 2023 13:57:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH v2 0/3] KVM: x86: SGX vs. XCR0 cleanups
On Fri, May 19, 2023, Paolo Bonzini wrote:
> On 5/3/23 18:08, Sean Christopherson wrote:
> > Stop adjusting the guest's CPUID info for the allowed XFRM (a.k.a. XCR0)
> > for SGX enclaves. Past me didn't understand the roles and responsibilities
> > between userspace and KVM with respect to CPUID leafs, i.e. I thought I was
> > being helpful by having KVM adjust the entries.
> >
> > This is clearly an ABI change, but QEMU does the right thing and AFAIK no
> > other VMMs support SGX (yet), so I'm hopeful/confident that we can excise
> > the ugly before userspace starts depending on the bad behavior.
> > v2:
> > - Collect reviews/testing. [Kai]
> > - Require FP+SSE to always be set in XFRM, and exempt them from the XFRM
> > vs. XCR0 check. [Kai]
> >
> > v1: https://lore.kernel.org/all/20230405005911.423699-1-seanjc@google.com
> >
> > Sean Christopherson (3):
> > KVM: VMX: Don't rely _only_ on CPUID to enforce XCR0 restrictions for
> > ECREATE
> > KVM: x86: Don't adjust guest's CPUID.0x12.1 (allowed SGX enclave XFRM)
> > KVM: x86: Open code supported XCR0 calculation in
> > kvm_vcpu_after_set_cpuid()
> >
> > arch/x86/kvm/cpuid.c | 43 ++++++++++--------------------------------
> > arch/x86/kvm/vmx/sgx.c | 11 +++++++++--
> > 2 files changed, 19 insertions(+), 35 deletions(-)
> >
> >
> > base-commit: 5c291b93e5d665380dbecc6944973583f9565ee5
>
> Queued, thanks. But why patch 3?
I want to guard against future misuse of calculating the support XCR0 before the
CPUID update is complete. I suppose I could have done this:
static u64 guest_cpuid_supported_xcr0(struct kvm_vcpu *vcpu)
{
struct kvm_cpuid_entry2 *best;
best = kvm_find_cpuid_entry_index(vcpu, 0xd, 0);
if (!best)
return 0;
return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
}
but I don't really see the point since there should only ever be one caller,
e.g. unlike cpuid_query_maxphyaddr() which needs a non-zero default.
> Small functions are nice and remove the need to remember what is in EDX:EAX
> of CPUID[0xD,0].
Hmm, yes and no. Specifically, I dislike single-use helpers that bury unintuitive
details in the helper, e.g. in this case, filtering the raw iguest CPUID with KVM's
kvm_caps.supported_xcr0. Communicating that in the name of the function so that
they're are no surprises is often more difficult than just open coding things.
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