linux-kernel - Re: [RFC PATCH 13/22] x86/fpu/xstate: Expand dynamic user state area on first use

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Date:   Tue, 3 Nov 2020 21:32:12 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     "Lutomirski, Andy" <luto@...capital.net>,
        "Hansen, Dave" <dave.hansen@...el.com>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...e.de" <bp@...e.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Brown, Len" <len.brown@...el.com>
Subject: Re: [RFC PATCH 13/22] x86/fpu/xstate: Expand dynamic user state area
 on first use

On Wed, 2020-10-14 at 09:29 -0700, Dave Hansen wrote:
> On 10/14/20 9:10 AM, Andy Lutomirski wrote:
> > 
> > I was suggesting something a little bit different.  We'd keep XMM,
> > YMM, ZMM, etc state stored exactly the way we do now and, for
> > AMX-using tasks, we would save the AMX state in an entirely separate
> > buffer.  This way the pain of having a variable xstate layout is
> > confined just to AMX tasks.
> 
> OK, got it.
> 
> So, we'd either need a second set of XSAVE/XRSTORs, or "manual" copying
> of the registers out to memory.  We can preserve the modified
> optimization if we're careful about ordering, but only for *ONE* of the
> XSAVE buffers (if we use two).

For what is worth,

If using two buffers, the buffer for saving the tile data also needs space
for the legacy states.

The AMX state is stored at the end of the XSAVE buffer (at least for now).
So, the layout (in terms of offsets of non-AMX states) won't be changed at
run-time.

Thanks,
Chang