linux-kernel - Re: [PATCH v10 035/108] KVM: x86/mmu: Track shadow MMIO value on a per-VM basis

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Date:   Fri, 25 Nov 2022 00:45:01 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v10 035/108] KVM: x86/mmu: Track shadow MMIO value on a
 per-VM basis

On Fri, 2022-11-25 at 08:12 +0800, Yan Zhao wrote:
> On Fri, Nov 25, 2022 at 08:13:48AM +0800, Huang, Kai wrote:
> > On Tue, 2022-11-22 at 10:10 +0800, Yan Zhao wrote:
> > > Also make enable_mmio_caching to be a per-VM value?
> > > As if the shadow_mmio_value is 0, mmio_caching needs to be disabled.
> > 
> > If I recall correctly, Sean said we can disable TDX guests if mmio_caching is
> > disabled (we also will need to change to allow enable_mmio_caching to still be
> > true when mmio_value is 0).
> > 
> > SEV_ES has similar logic:
> > 
> > void __init sev_hardware_setup(void)
> > {
> > 
> > 	...
> > 
> >         /*
> >          * SEV-ES requires MMIO caching as KVM doesn't have access to the guest
> >          * instruction stream, i.e. can't emulate in response to a #NPF and
> >          * instead relies on #NPF(RSVD) being reflected into the guest as #VC
> >          * (the guest can then do a #VMGEXIT to request MMIO emulation).
> >          */
> >         if (!enable_mmio_caching)
> >                 goto out;
> > 
> 
> Would enabling mmio caching in per-VM basis be better?
> 

We need Paolo/Sean to decide.

The thing is TDX guests always require mmio_caching being enabled.  For VMX
guests, normally we will always enable mmio_caching too.  So I think per-VM
basis mmio_caching is not that useful.