linux-kernel - Re: [PATCH] x86: svm: don't intercept CR0 TS or MP bit write

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Message-ID: <54ECEC2F.1090700@amd.com>
Date:	Tue, 24 Feb 2015 15:25:03 -0600
From:	Joel Schopp <joel.schopp@....com>
To:	Radim Krčmář <rkrcmar@...hat.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] x86: svm: don't intercept CR0 TS or MP bit write


>> -		clr_cr_intercept(svm, INTERCEPT_CR0_WRITE);
>>  	} else {
>>  		set_cr_intercept(svm, INTERCEPT_CR0_READ);
> (There is no point in checking fpu_active if cr0s are equal.)
>
>> -		set_cr_intercept(svm, INTERCEPT_CR0_WRITE);
> KVM uses lazy FPU and the state is undefined before the first access.
> We set cr0.ts when !svm->vcpu.fpu_active to detect the first access, but
> if we allow the guest to clear cr0.ts without exiting, it can access FPU
> with undefined state.
Thanks for the valuable feedback.  It's apparent I hadn't thought
through the interaction with lazy FPU and will need to go back and
rethink my approach here.
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