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From: Borislav Petkov <bp@alien8.de>
To: Thiago Macieira <thiago.macieira@intel.com>
Cc: "Chang S. Bae" <chang.seok.bae@intel.com>,
	luto@kernel.org, tglx@linutronix.de, mingo@kernel.org,
	x86@kernel.org, len.brown@intel.com, dave.hansen@intel.com,
	jing2.liu@intel.com, ravi.v.shankar@intel.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state
Date: Wed, 18 Aug 2021 20:10:10 +0200	[thread overview]
Message-ID: <YR1NAvJW4w8bhEEu@zn.tnic> (raw)
In-Reply-To: <4493449.UzBjrsCbmA@tjmaciei-mobl5>

On Wed, Aug 18, 2021 at 10:58:42AM -0700, Thiago Macieira wrote:
> That tells me what the CPU supports, not what the kernel does. By
> omitting the "xfd" entry in /proc/cpuinfo, we are assuming that all
> kernels with "amxtile" also implicitly support xfd. That is a valid
> assumption.

What relevance does the fact have for userspace whether the kernel
supports XFD or not?

IOW, userspace cares about AMX and the other features which are supposed
to use XFD - not how those features are implemented: whether with
faulting or with pre-allocation or whatever.

> Many applications need to determine which plugins and code paths to
> enable before getting the data that will tell them what to do. It's
> entirely possible for them to never need to run the AMX instructions,
> so they may wish to defer the request to allocate the XSAVE state
> until they have read their input data.
>
> It's indeed possible that the allocation then fails and the
> application be unable to continue. But OOM conditions are unlikely, so
> it may be an acceptable price to pay. In fact, by *not* allocating the
> extra state for every thread in the current process, it may avoid the
> OOM.

And?

That doesn't conflict with my suggestion. It goes and asks the kernel
what it supports and then requests the buffers.

> Sorry, that's not what I meant. I was going to request an extra API, a third 
> call. We'd have:
>  - get current state
>  - set new state
>  - get available bits to set

Yes, this should have been the API from the very beginning. Of course
you need to be able to query what bits can be set at all.

> ...
> Now, if we are going to have this API any way, it might be a good
> idea to combine the two getters in one by adding a second pointer
> parameter.

Yeah, I'll get to that patch in the coming days and have a look. So far,
it only makes sense to have a querying API too so that we can provide
support for more "fat" features.

Unless Intel folks decide to stop using XSAVE for that - it was a bad
idea in the first place anyway, TBH - but it's not like hw people listen
to sw folk so...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

  reply	other threads:[~2021-08-18 18:09 UTC|newest]

Thread overview: 91+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30 14:59 [PATCH v9 00/26] x86: Support Intel Advanced Matrix Extensions Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 01/26] x86/fpu/xstate: Modify the initialization helper to handle both static and dynamic buffers Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 02/26] x86/fpu/xstate: Modify state copy helpers " Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 03/26] x86/fpu/xstate: Modify address finders " Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 04/26] x86/fpu/xstate: Add a new variable to indicate dynamic user states Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 05/26] x86/fpu/xstate: Add new variables to indicate dynamic XSTATE buffer size Chang S. Bae
2021-08-12 15:03   ` Borislav Petkov
2021-07-30 14:59 ` [PATCH v9 06/26] x86/fpu/xstate: Calculate and remember dynamic XSTATE buffer sizes Chang S. Bae
2021-08-12 16:36   ` Borislav Petkov
2021-07-30 14:59 ` [PATCH v9 07/26] x86/fpu/xstate: Convert the struct fpu 'state' field to a pointer Chang S. Bae
2021-08-12 17:09   ` Borislav Petkov
2021-07-30 14:59 ` [PATCH v9 08/26] x86/fpu/xstate: Introduce helpers to manage the XSTATE buffer dynamically Chang S. Bae
2021-08-12 19:44   ` Borislav Petkov
2021-08-13  8:04     ` Bae, Chang Seok
2021-08-13 10:04       ` Borislav Petkov
2021-08-13 19:43         ` Bae, Chang Seok
2021-08-18  9:28           ` Borislav Petkov
2021-08-18 19:46             ` Bae, Chang Seok
2021-08-25 16:01               ` Bae, Chang Seok
2021-08-30 17:07               ` Borislav Petkov
2021-08-30 23:39                 ` Bae, Chang Seok
2021-08-16 18:33     ` Bae, Chang Seok
2021-08-16 18:53       ` Borislav Petkov
2021-08-30 17:45   ` Dave Hansen
2021-08-30 23:39     ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 09/26] x86/fpu/xstate: Update the XSTATE save function to support dynamic states Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 10/26] x86/fpu/xstate: Update the XSTATE buffer address finder " Chang S. Bae
2021-08-18 11:33   ` Borislav Petkov
2021-08-18 19:47     ` Bae, Chang Seok
2021-08-30 17:18       ` Borislav Petkov
2021-08-30 23:38         ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 11/26] x86/fpu/xstate: Update the XSTATE context copy function " Chang S. Bae
2021-08-18 12:03   ` Borislav Petkov
2021-08-18 19:47     ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state Chang S. Bae
2021-08-18 16:24   ` Borislav Petkov
2021-08-18 17:20     ` Thiago Macieira
2021-08-18 17:46       ` Borislav Petkov
2021-08-18 17:58         ` Thiago Macieira
2021-08-18 18:10           ` Borislav Petkov [this message]
2021-08-24 22:51             ` Len Brown
2021-08-18 20:43         ` Bae, Chang Seok
2021-08-18 21:04           ` Thiago Macieira
2021-08-18 21:12             ` Bae, Chang Seok
2021-08-18 22:27               ` Thiago Macieira
2021-08-19  1:21             ` Andy Lutomirski
2021-08-19 16:06               ` Thiago Macieira
2021-08-18 21:17           ` Borislav Petkov
2021-08-18 21:37             ` Bae, Chang Seok
2021-08-19  8:00               ` Borislav Petkov
2021-08-19 15:24                 ` Bae, Chang Seok
2021-08-24 23:22             ` Len Brown
2021-08-30 17:31               ` Borislav Petkov
2021-09-17  3:48                 ` Len Brown
2021-08-18 19:47     ` Bae, Chang Seok
2021-08-24 22:21     ` Len Brown
2021-08-30 17:41       ` Borislav Petkov
2021-08-31 21:44         ` Len Brown
2021-08-24 23:17     ` Len Brown
2021-08-30 17:53       ` Borislav Petkov
2021-08-31 22:07         ` Len Brown
2021-08-31 22:11           ` Dave Hansen
2021-08-30 18:04       ` Dave Hansen
2021-08-31 22:15         ` Len Brown
2021-08-31 22:16           ` Len Brown
2021-08-31 22:39           ` Thiago Macieira
2021-08-31 22:44             ` Len Brown
2021-07-30 14:59 ` [PATCH v9 13/26] x86/fpu/xstate: Support ptracer-induced XSTATE buffer expansion Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 14/26] x86/arch_prctl: Create ARCH_SET_STATE_ENABLE/ARCH_GET_STATE_ENABLE Chang S. Bae
2021-08-06 16:46   ` Thiago Macieira
2021-08-09 22:08     ` Bae, Chang Seok
2021-08-09 23:42       ` Thiago Macieira
2021-08-10  0:57         ` Bae, Chang Seok
2021-08-13 19:44           ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 15/26] x86/fpu/xstate: Support both legacy and expanded signal XSTATE size Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 16/26] x86/fpu/xstate: Adjust the XSAVE feature table to address gaps in state component numbers Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 17/26] x86/fpu/xstate: Disable XSTATE support if an inconsistent state is detected Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 18/26] x86/cpufeatures/amx: Enumerate Advanced Matrix Extension (AMX) feature bits Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 19/26] x86/fpu/amx: Define AMX state components and have it used for boot-time checks Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 20/26] x86/fpu/amx: Initialize child's AMX state Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 21/26] x86/fpu/amx: Enable the AMX feature in 64-bit mode Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 22/26] x86/fpu/xstate: Skip writing zeros to signal frame for dynamic user states if in INIT-state Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 23/26] selftest/x86/amx: Test cases for the AMX state management Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 24/26] x86/insn/amx: Add TILERELEASE instruction to the opcode map Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 25/26] intel_idle/amx: Add SPR support with XTILEDATA capability Chang S. Bae
2021-07-30 18:41   ` Dave Hansen
2021-08-03 21:32     ` Bae, Chang Seok
2021-08-03 21:38       ` Dave Hansen
2021-08-03 21:43         ` Brown, Len
2021-07-30 20:15   ` Dave Hansen
2021-07-30 14:59 ` [PATCH v9 26/26] x86/fpu/xstate: Add a sanity check for XFD state when saving XSTATE Chang S. Bae

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