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Subject: Re: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>
References: <20200622220442.21998-1-peterx@redhat.com>
 <20200622220442.21998-2-peterx@redhat.com>
 <20200625061544.GC2141@linux.intel.com>
 <1cebc562-89e9-3806-bb3c-771946fc64f3@redhat.com>
 <20200625162540.GC3437@linux.intel.com> <20200626180732.GB175520@xz-x1>
 <20200626181820.GG6583@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <47b90b77-cf03-6087-b25f-fcd2fd313165@redhat.com>
Date:   Sat, 27 Jun 2020 16:24:34 +0200
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On 26/06/20 20:18, Sean Christopherson wrote:
>> Btw, would it be more staightforward to check "vcpu->arch.arch_capabilities &
>> ARCH_CAP_TSX_CTRL_MSR" rather than "*ebx | (F(RTM) | F(HLE))" even if we want
>> to have such a fix?
> Not really, That ends up duplicating the check in vmx_get_msr().  From an
> emulation perspective, this really is a "guest" access to the MSR, in the
> sense that it the virtual CPU is in the guest domain, i.e. not a god-like
> entity that gets to break the rules of emulation.

But if you wrote a guest that wants to read MSR_IA32_TSX_CTRL, there are
two choices:

1) check ARCH_CAPABILITIES first

2) blindly access it and default to 0.

Both are fine, because we know MSR_IA32_TSX_CTRL has no
reserved/must-be-one bits.  Calling __kvm_get_msr and checking for an
invalid MSR through the return value is not breaking the rules of
emulation, it is "faking" a #GP handler.

So I think Peter's patch is fine, but (possibly on top as a third patch)
__must_check should be added to MSR getters and setters.  Also one
possibility is to return -EINVAL for invalid MSRs.

Paolo