linux-kernel - RE: [PATCH v2 04/25] KVM: x86: Mark CR4.FRED as not reserved

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Date: Thu, 13 Jun 2024 03:27:48 +0000
From: "Li, Xin3" <xin3.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "linux-kselftest@...r.kernel.org"
	<linux-kselftest@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "corbet@....net" <corbet@....net>,
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	"peterz@...radead.org" <peterz@...radead.org>, "Shankar, Ravi V"
	<ravi.v.shankar@...el.com>, "xin@...or.com" <xin@...or.com>
Subject: RE: [PATCH v2 04/25] KVM: x86: Mark CR4.FRED as not reserved


> On Wed, Feb 07, 2024, Xin Li wrote:
> > The CR4.FRED bit, i.e., CR4[32], is no longer a reserved bit when a guest
> > enumerates FRED, otherwise it is still a reserved bit.
> 
> This isn't quite correct, as __cr4_reserved_bits() is used with kvm_cpu_caps too,
> i.e. to compute CR4 bits that are reserved from the host's perspective.  And that
> matters, because if this check was done _only_ on guest CPUID, then KVM would
> allow CR4.FRED=1 before all of KVM support is in place.

Ah, that means I didn't dig deep enough.

Thanks!
    Xin