linux-kernel - Re: [PATCH] kvm: x86: svm: remove SVM_EXIT_READ

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Date:	Thu, 12 Mar 2015 22:20:02 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Joel Schopp <joel.schopp@....com>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
	linux-kernel@...r.kernel.org, David Kaplan <david.kaplan@....com>
Subject: Re: [PATCH] kvm: x86: svm: remove SVM_EXIT_READ_CR* intercepts

2015-03-12 15:17-0500, Joel Schopp:
> There isn't really a valid reason for kvm to intercept cr* reads
> on svm hardware.  The current kvm code just ends up returning
> the register

There is no need to intercept CR* if the value that the guest should see
is equal to what we set there, but that is not always the case:
- CR0 might differ from what the guest should see because of lazy fpu
- CR3 isn't intercepted with nested paging and it should differ
  otherwise
- CR4 contains PAE bit when run without nested paging

CR2 and CR8 already aren't intercepted, so it looks like only CR0 and
CR4 could use some optimizations.
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