linux-kernel - Re: [PATCH v10 047/108] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases

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Date:   Thu, 15 Dec 2022 23:03:59 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
CC:     "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v10 047/108] KVM: x86/tdp_mmu: Don't zap private pages for
 unsupported cases

On Thu, 2022-12-15 at 14:46 -0800, Isaku Yamahata wrote:
> > Btw, as you mentioned in the changelog, private memory always has WB memory
> > type, thus cannot be virtualized.  Is it better to modify update_mtrr() to
> > just
> > return early if the gfn range is purely private?
> 
> MTRR support in cpuid is fixed to 1, PAT in cpuid is native.
> MTRR and PAT are supported on shared pages.
> 

But none of those are mentioned in the changelog or whatever.  They are hidden
in the spec or in the later patches far away.  

ALso, the handling of "load/save IA32_PAT" in VMEXIT/VMENTRY VMCS control is
different between TDX module and KVM.  None of those are mentioned.

All those make the patch review so hard.

> 
> > IMHO the handling of MTRR/PAT virtualization for TDX guest deserves
> > dedicated
> > patch(es) to put them together so it's easier to review.  Now the relevant
> > parts
> > spread in multiple independent patches (MSR handling, vt_get_mt_mask(),
> > etc).
> 
> Ok, let me check it.

Yes.  IMHO you should put all relevant parts together and make a clear changelog
to justify the patch(es).