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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state
Date:   Wed, 24 Mar 2021 14:42:28 -0700
Message-Id: <03F61C1A-0CA1-43BB-B2B5-C9D654943051@amacapital.net>
References: <57fafdfe-0c2c-a5b5-a2b9-744fb9d34be8@intel.com>
Cc:     Len Brown <lenb@kernel.org>,
        "Liu, Jing2" <jing2.liu@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@suse.de>, Ingo Molnar <mingo@kernel.org>,
        X86 ML <x86@kernel.org>, Len Brown <len.brown@intel.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>
In-Reply-To: <57fafdfe-0c2c-a5b5-a2b9-744fb9d34be8@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Precedence: bulk


> On Mar 24, 2021, at 2:30 PM, Dave Hansen <dave.hansen@intel.com> wrote:
>=20
> =EF=BB=BFOn 3/24/21 2:26 PM, Andy Lutomirski wrote:
>>> 3. user space always uses fully uncompacted XSAVE buffers.
>>>=20
>> There is no reason we have to do this for new states. Arguably we
>> shouldn=E2=80=99t for AMX to avoid yet another altstack explosion.
>=20
> The thing that's worried me is that the list of OS-enabled states is
> visible to apps via XGETBV.  It doesn't seem too much of a stretch to
> think that apps will see AMX enabled with XGETBV and them assume that
> it's on the signal stack.
>=20
> Please tell me I'm being too paranoid.  If we can break this assumption,
> it would get rid of a lot of future pain.

There are no AMX apps. I sure hope that there are no apps that enumerate xfe=
atures with CPUID and try to decode the mess in the signal stack.

I do think we need to save AMX state *somewhere* if a signal happens unless u=
serspace opts out, but I don=E2=80=99t think it needs to be in the nominally=
 expected spot.=