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From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "George Dunlap" <george.dunlap@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v3 05/22] x86/xstate: drop xstate_offsets[] and xstate_sizes[]
Date: Tue, 4 May 2021 09:57:47 +0200	[thread overview]
Message-ID: <ebf0945a-db78-66de-2f64-860c5067220d@suse.com> (raw)
In-Reply-To: <f3a9b372-c927-70e3-a2ba-fef2bb2c7d7a@citrix.com>

On 03.05.2021 18:10, Andrew Cooper wrote:
> On 22/04/2021 15:45, Jan Beulich wrote:
>> They're redundant with respective fields from the raw CPUID policy; no
>> need to keep two copies of the same data.
> 
> So before I read this patch of yours, I had a separate cleanup patch
> turning the two arrays into static const.
> 
>> This also breaks
>> recalculate_xstate()'s dependency on xstate_init(),
> 
> It doesn't, because you've retained the reference to xstate_align, which
> is calculated in xstate_init().

Good point - s/breaks/eliminates some of/.

>  I've posted "[PATCH 4/5] x86/cpuid:
> Simplify recalculate_xstate()" which goes rather further.

I'll see to take a look soonish.

> xstate_align, and xstate_xfd as you've got later in the series, don't
> need to be variables.  They're constants, just like the offset/size
> information, because they're all a description of the XSAVE ISA
> instruction behaviour.

Hmm, I think there are multiple views possible - for xfd_mask even more
than for xstate_align: XFD is, according to my understanding of the
spec, not a prereq feature to AMX. IOW AMX would function fine without
XFD, just that lazy state saving space allocation then wouldn't be
possible. And I also can't, in principle, see any reason why largish
components like the AVX512 ones couldn't become XFD-sensitive (in
hardware, we of course can't mimic this in software).

(I could take as proof sde reporting AMX but not XFD with -spr, but I
rather suspect this to be an oversight in their CPUID data. I've posted
a respective question in their forum.)

If there really was a strict static relationship, I'm having trouble
seeing why the information would need expressing in CPUID at all. It
would at least feel like over-engineering then.

> We never turn on states we don't understand, which means we don't
> actually need to refer to any component subleaf, other than to cross-check.
> 
> I'm still on the fence as to whether it is better to compile in the
> constants, or to just use the raw policy.  Absolutely nothing good will
> come of the constants changing, and one of my backup plans for dealing
> with the size of cpuid_policy if it becomes a problem was to not store
> these leaves, and generate them dynamically on request.

Actually it is my understanding that the reason the offsets are
expressed via CPUID is that originally it was meant for them to be
able to vary between implementations (see in particular the placement
of the LWP component, which has resulted in a curious 128-byte gap
ahead of the MPX components). Until it was realized what implications
this would have on migration.

>> allowing host CPUID
>> policy calculation to be moved together with that of the raw one (which
>> a subsequent change will require anyway).
> 
> While breaking up the host/raw calculations from the rest, we really
> need to group the MSR policy calculations with their CPUID counterparts.

But that's orthogonal to the change here, i.e. if at all for this
series subject of a separate patch. Plus I have to admit I'm not
sure I see what your plan here would be - cpuid.c and msr.c so far
don't cross reference one another. And I thought this separation
was intentional.

Jan


  reply	other threads:[~2021-05-04  7:58 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-22 14:38 [PATCH v3 00/22] xvmalloc() / x86 xstate area / x86 CPUID / AMX+XFD Jan Beulich
2021-04-22 14:43 ` [PATCH v3 01/22] mm: introduce xvmalloc() et al and use for grant table allocations Jan Beulich
2021-05-03 11:31   ` Roger Pau Monné
2021-05-03 13:50     ` Jan Beulich
2021-05-03 14:54       ` Roger Pau Monné
2021-05-03 15:21         ` Jan Beulich
2021-05-03 16:39           ` Roger Pau Monné
2021-04-22 14:44 ` [PATCH v3 02/22] x86/xstate: use xvzalloc() for save area allocation Jan Beulich
2021-05-05 13:29   ` Roger Pau Monné
2021-04-22 14:44 ` [PATCH v3 03/22] x86/xstate: re-size save area when CPUID policy changes Jan Beulich
2021-05-03 13:57   ` Andrew Cooper
2021-05-03 14:22     ` Jan Beulich
2021-05-11 16:41       ` Andrew Cooper
2021-05-17  7:33         ` Jan Beulich
2021-04-22 14:45 ` [PATCH v3 04/22] x86/xstate: re-use valid_xcr0() for boot-time checks Jan Beulich
2021-05-03 11:53   ` Andrew Cooper
2021-04-22 14:45 ` [PATCH v3 05/22] x86/xstate: drop xstate_offsets[] and xstate_sizes[] Jan Beulich
2021-05-03 16:10   ` Andrew Cooper
2021-05-04  7:57     ` Jan Beulich [this message]
2021-04-22 14:46 ` [PATCH v3 06/22] x86/xstate: replace xsave_cntxt_size and drop XCNTXT_MASK Jan Beulich
2021-04-22 14:47 ` [PATCH v3 07/22] x86/xstate: avoid accounting for unsupported components Jan Beulich
2021-04-22 14:47 ` [PATCH v3 08/22] x86: use xvmalloc() for extended context buffer allocations Jan Beulich
2021-04-22 14:48 ` [PATCH v3 09/22] x86/xstate: enable AMX components Jan Beulich
2021-04-22 14:50 ` [PATCH v3 10/22] x86/CPUID: adjust extended leaves out of range clearing Jan Beulich
2021-04-22 14:50 ` [PATCH v3 11/22] x86/CPUID: move bounding of max_{,sub}leaf fields to library code Jan Beulich
2021-04-22 14:51 ` [PATCH v3 12/22] x86/CPUID: enable AMX leaves Jan Beulich
2021-04-22 14:52 ` [PATCH v3 13/22] x86: XFD enabling Jan Beulich
2021-04-22 14:53 ` [PATCH v3 14/22] x86emul: introduce X86EMUL_FPU_{tilecfg,tile} Jan Beulich
2021-04-22 14:53 ` [PATCH v3 15/22] x86emul: support TILERELEASE Jan Beulich
2021-04-22 14:53 ` [PATCH v3 16/22] x86: introduce struct for TILECFG register Jan Beulich
2021-04-22 14:54 ` [PATCH v3 17/22] x86emul: support {LD,ST}TILECFG Jan Beulich
2021-04-22 14:55 ` [PATCH v3 18/22] x86emul: support TILEZERO Jan Beulich
2021-04-22 14:55 ` [PATCH v3 19/22] x86emul: support TILELOADD{,T1} and TILESTORE Jan Beulich
2021-04-22 15:06   ` Jan Beulich
2021-04-22 15:11     ` Jan Beulich
2021-04-26  7:12       ` Paul Durrant
2021-04-29  9:40         ` Jan Beulich
2021-04-22 14:56 ` [PATCH v3 20/22] x86emul: support tile multiplication insns Jan Beulich
2021-04-22 14:57 ` [PATCH v3 21/22] x86emul: test AMX insns Jan Beulich
2021-04-22 14:57 ` [PATCH v3 22/22] x86: permit guests to use AMX and XFD Jan Beulich

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