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

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Message-ID: <20150225202600.GA11660@potion.brq.redhat.com>
Date:	Wed, 25 Feb 2015 21:26:01 +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] x86: svm: don't intercept CR0 TS or MP bit write

2015-02-24 15:25-0600, Joel Schopp:
> 
> >> -		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.

I don't think we can gain much without sacrificing some laziness, like:
when a guest with lazy FPU clears CR0.TS, it is going to use that FPU,
so we could pre-load FPU in this case and drop the write intercept too;
guests that unconditionally clear CR0.TS would perform worse though.

It's going to take a lot of time, but two hunks in your patch, that made
selective intercept benefit from decode assists, look useful even now.

Would you post them separately?

Thanks.
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