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

From: Yu-cheng Yu
Date: Wed May 04 2016 - 18:50:49 EST

On Wed, May 04, 2016 at 03:41:49PM -0700, Dave Hansen wrote:
> 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.

In the coming version 5 patches, we are going to have one additional
patch for fixing __fpu_restore_sig() for the compacted format. I changed
my existing patch a little and run into some problems. Fixing it now.

Our ptrace tests went OK before, but are failing now. It might be relating
to what you are saying? I will check it.