linux-kernel - Re: [PATCH 17/21] KVM: TDX: MTRR: implement get_mt

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Message-ID: <ZuBSNS33_ck-w6-9@google.com>
Date: Tue, 10 Sep 2024 07:05:41 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, kvm@...r.kernel.org, kai.huang@...el.com, 
	dmatlack@...gle.com, isaku.yamahata@...il.com, yan.y.zhao@...el.com, 
	nik.borisov@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 17/21] KVM: TDX: MTRR: implement get_mt_mask() for TDX

On Tue, Sep 10, 2024, Paolo Bonzini wrote:
> On 9/4/24 05:07, Rick Edgecombe wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > Although TDX supports only WB for private GPA, it's desirable to support
> > MTRR for shared GPA.  Always honor guest PAT for shared EPT as what's done
> > for normal VMs.
> > 
> > Suggested-by: Kai Huang <kai.huang@...el.com>
> > Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> > ---
> > TDX MMU part 2 v1:
> >   - Align with latest vmx code in kvm/queue.
> >   - Updated patch log.
> >   - Dropped KVM_BUG_ON() in vt_get_mt_mask(). (Rick)
> 
> The only difference at this point is
> 
>         if (!static_cpu_has(X86_FEATURE_SELFSNOOP) &&
>             !kvm_arch_has_noncoherent_dma(vcpu->kvm))
>                 return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) |
> VMX_EPT_IPAT_BIT;
> 
> 
> which should never be true.  I think this patch can simply be dropped.

And we can/should do what we've done for SEV, and make it a hard dependency to
enable TDX, e.g. similar to this:

	/*
	 * SEV must obviously be supported in hardware.  Sanity check that the
	 * CPU supports decode assists, which is mandatory for SEV guests to
	 * support instruction emulation.  Ditto for flushing by ASID, as SEV
	 * guests are bound to a single ASID, i.e. KVM can't rotate to a new
	 * ASID to effect a TLB flush.
	 */
	if (!boot_cpu_has(X86_FEATURE_SEV) ||
	    WARN_ON_ONCE(!boot_cpu_has(X86_FEATURE_DECODEASSISTS)) ||
	    WARN_ON_ONCE(!boot_cpu_has(X86_FEATURE_FLUSHBYASID)))
		goto out;