From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Andy Lutomirski <luto@kernel.org>, Florian Weimer <fw@deneb.enyo.de>
Cc: "H. J. Lu" <hjl.tools@gmail.com>, X86 ML <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"Bae, Chang Seok" <chang.seok.bae@intel.com>,
Carlos O'Donell <carlos@redhat.com>,
Rich Felker <dalias@libc.org>,
libc-alpha <libc-alpha@sourceware.org>
Subject: Re: Why does glibc use AVX-512?
Date: Fri, 26 Mar 2021 20:06:57 +0000 [thread overview]
Message-ID: <25dd7d39-ce7b-2f4e-a6eb-981038f74c9a@citrix.com> (raw)
In-Reply-To: <CALCETrUXMnutTV=SEs6ot58j32_5=K5Z=G7_57gVZt_GFcuDiw@mail.gmail.com>
On 26/03/2021 19:47, Andy Lutomirski wrote:
> On Fri, Mar 26, 2021 at 12:34 PM Florian Weimer <fw@deneb.enyo.de> wrote:
>> * Andy Lutomirski:
>>
>>>>> AVX-512 cleared, and programs need to explicitly request enablement.
>>>>> This would allow programs to opt into not saving/restoring across
>>>>> signals or to save/restore in buffers supplied when the feature is
>>>>> enabled.
>>>> Isn't XSAVEOPT already able to handle that?
>>>>
>>> Yes, but we need a place to put the data, and we need to acknowledge
>>> that, with the current save-everything-on-signal model, the amount of
>>> time and memory used is essentially unbounded. This isn't great.
>> The size has to have a known upper bound, but the save amount can be
>> dynamic, right?
>>
>> How was the old lazy FPU initialization support for i386 implemented?
>>
>>>> Assuming you can make XSAVEOPT work for you on the kernel side, my
>>>> instincts tell me that we should have markup for RTM, not for AVX-512.
>>>> This way, we could avoid use of the AVX-512 registers and keep using
>>>> VZEROUPPER, without run-time transaction checks, and deal with other
>>>> idiosyncrasies needed for transaction support that users might
>>>> encounter once this feature sees more use. But the VZEROUPPER vs RTM
>>>> issues is currently stuck in some internal process issue on my end (or
>>>> two, come to think of it), which I hope to untangle next month.
>>> Can you elaborate on the issue?
>> This is the bug:
>>
>> vzeroupper use in AVX2 multiarch string functions cause HTM aborts
>> <https://sourceware.org/bugzilla/show_bug.cgi?id=27457>
>>
>> Unfortunately we have a bug (outside of glibc) that makes me wonder if
>> we can actually roll out RTM transaction checks (or any RTM
>> instruction) on a large scale:
>>
>> x86: Sporadic failures in tst-cpu-features-cpuinfo
>> <https://sourceware.org/bugzilla/show_bug.cgi?id=27398#c3>
> It's worth noting that recent microcode updates have make RTM
> considerably less likely to actually work on many parts. It's
> possible you should just disable it. :(
For a variety of errata and speculative security reasons, hypervisors
now have the ability to hide/show the HLE/RTM CPUID bits, independently
of letting TSX actually work or not.
For migration compatibility reasons, you might quite possibly find
yourself in a VM which advertises the HLE/RTM bits but will
unconditionally abort any transaction.
Honestly, if I were you, I'd just leave it to the user to explicitly opt
in if they want transactions.
~Andrew
next prev parent reply other threads:[~2021-03-26 20:07 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-26 4:38 Andy Lutomirski
2021-03-26 10:06 ` Borislav Petkov
2021-03-26 18:17 ` Andy Lutomirski
2021-03-26 12:12 ` Florian Weimer
2021-03-26 18:14 ` Andy Lutomirski
2021-03-26 19:34 ` Florian Weimer
2021-03-26 19:47 ` Andy Lutomirski
2021-03-26 20:06 ` Andrew Cooper [this message]
2021-03-26 20:35 ` Florian Weimer
2021-03-26 20:43 ` H.J. Lu
2021-03-26 20:48 ` Andy Lutomirski
2021-03-26 21:11 ` Florian Weimer
2021-03-26 21:21 ` Andy Lutomirski
2021-03-26 13:32 ` David Laight
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