linux-kernel - Re: [PATCH v4 10/10] x86/xsaves: Re-enable XSAVES

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Date:	Wed, 4 May 2016 15:41:49 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v4 10/10] x86/xsaves: Re-enable XSAVES

It's my fault, but you also need to go update

	fpu__xfeature_set_state()
and
	__raw_xsave_addr()

The theoretical problem is that you might ask for a __raw_xsave_addr()
of a component which has been compacted out of an XSAVES buffer and thus
has no address.  We could work around this by doing a memmove() and
moving the components "up" after the one we are trying to set in order
to make space.

But, since we *always* call XSAVES with an instruction mask of -1 and
end up with a requested feature bitmap (RFBM) equal to XCR0, I think we
can do a shortcut because we'll practically *always* have an
xcomp_bv==RFBM==XCR0, which means that all (present) components will
always have an address.

So, the alternative to doing the memmove() is to add some WARN_ON_FPU()
checks to enforce xcomp_bv==RFBM==XCR0 in places where we call
XSAVES/XRSTORS and __raw_xsave_addr(), maybe more.