linux-kernel - Re: [PATCH 1/2] KVM: X86: Move ignore

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Date:   Thu, 9 Jul 2020 14:22:20 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack

On Tue, Jun 30, 2020 at 08:47:26AM -0700, Sean Christopherson wrote:
> On Sat, Jun 27, 2020 at 04:24:34PM +0200, Paolo Bonzini wrote:
> > 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.
> 
> "guest" was the wrong choice of word.  My point was that, IMO, emulation
> should never set host_initiated=true.
> 
> To me, accessing MSRs with host_initiated is the equivalent of loading a
> ucode patch, i.e. it's super duper special stuff that deliberately turns
> off all safeguards and can change the fundamental behavior of the (virtual)
> CPU.

This seems to be an orthogonal change against what this series tried to do.  We
use host_initiated=true in current code, and this series won't change that fact
either.  As I mentioned in the other thread, at least the rdmsr warning is
ambiguous when it's not initiated from the guest if without this patchset, and
this series could address that.

> 
> > 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.

Yeah I can add another patch for that.  Also if to repost, I tend to also
introduce KVM_MSR_RET_[OK|ERROR] too, which seems to be cleaner when we had
KVM_MSR_RET_INVALID.

Any objections before I repost?

-- 
Peter Xu