linux-kernel - Re: How should we handle illegal task FPU state?

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Message-ID: <93999eea-468c-c4a7-d793-b6c82e4b26a4@citrix.com>
Date:   Thu, 1 Oct 2020 22:42:47 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
CC:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: How should we handle illegal task FPU state?

On 01/10/2020 21:58, Sean Christopherson wrote:
> On Thu, Oct 01, 2020 at 01:32:04PM -0700, Yu, Yu-cheng wrote:
>> On 10/1/2020 10:43 AM, Andy Lutomirski wrote:
>>> The question is: what do we do about it?  We have two basic choices, I think.
>>>
>>> a) Decide that the saved FPU for a task *must* be valid at all times.
>>> If there's a failure to restore state, kill the task.
>>>
>>> b) Improve our failed restoration handling and maybe even
>>> intentionally make it possible to create illegal state to allow
>>> testing.
>>>
>>> (a) sounds like a nice concept, but I'm not convinced it's practical.
>>> For example, I'm not even convinced that the set of valid SSP values
>>> is documented.
> Eh, crappy SDM writing isn't a good reason to make our lives harder.  The
> SSP MSRs are canonical MSRs and follow the same rules as the SYSCALL,
> FS/GS BASE, etc... MSRs.  I'll file an SDM bug.

Don't forget the added constraint of being 4 byte aligned. ;)

But the SDM is fine in this regard, at least as far as Vol4 goes, even
if does have an excessively verbose way of expressing itself.

~Andrew