linux-kernel - Re: [PATCH 2/4] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change

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Date:   Tue, 02 Aug 2022 08:46:26 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Michael Roth <michael.roth@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 2/4] KVM: x86/mmu: Fully re-evaluate MMIO caching when
 SPTE masks change

On Mon, 2022-08-01 at 14:15 +0000, Sean Christopherson wrote:
> On Mon, Aug 01, 2022, Kai Huang wrote:
> > On Fri, 2022-07-29 at 15:07 +0000, Sean Christopherson wrote:
> > > Lastly, in prepration for TDX, enable_mmio_caching should be changed to key off
> > > of the _mask_, not the value.  E.g. for TDX, the value will be '0', but the mask
> > > should be SUPPRESS_VE | RWX.
> > 
> > Agreed.  But perhaps in another patch.  We need to re-define what does
> > mask/value mean to enable_mmio_caching.
> 
> There's no need to redefine what they mean, the only change that needs to be made
> is handle the scenario where desire value is '0'.  Maybe that's all you mean by
> "redefine"?

My thinking is only when mask and value both are 0, enable_mmio_caching is
considered disabled.  vlaue=0 is valid when enable_mmio_caching is true as you
said.

> 
> Another thing to note is that only the value needs to be per-VM, the mask can be
> KVM-wide, i.e. "mask = SUPPRESS_VE | RWX" will work for TDX and non-TDX VMs when
> EPT is enabled.

Yeah, but is more like VMX and TDX both *happen* to have the same mask? 
Theoretically,  VMX only need RWX to trigger EPT misconfiguration but doesn't
need SUPPRESS_VE.

I don't see making mask/value both per-vm is a big issue?

-- 
Thanks,
-Kai