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From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
	Xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH 3/5] x86/xstate: Rework xstate_ctxt_size() as xstate_uncompressed_size()
Date: Tue, 4 May 2021 14:08:25 +0200	[thread overview]
Message-ID: <07a03e61-742c-5880-1003-fcded7efc662@suse.com> (raw)
In-Reply-To: <a8487667-1f47-1aae-1528-4a1224cbda7b@citrix.com>

On 03.05.2021 20:17, Andrew Cooper wrote:
> On 03/05/2021 16:39, Andrew Cooper wrote:
>> We're soon going to need a compressed helper of the same form.
>>
>> The size of the uncompressed image is a strictly a property of the highest
>> user state.  This can be calculated trivially with xstate_offsets/sizes, and
>> is much faster than a CPUID instruction in the first place, let alone the two
>> XCR0 writes surrounding it.
>>
>> Retain the cross-check with hardware in debug builds, but forgo it normal
>> builds.  In particular, this means that the migration paths don't need to mess
>> with XCR0 just to sanity check the buffer size.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> 
> The Qemu smoketests have actually found a bug here.
> 
> https://gitlab.com/xen-project/patchew/xen/-/jobs/1232118510/artifacts/file/smoke.serial
> 
> We call into xstate_uncompressed_size() from
> hvm_register_CPU_save_and_restore() so the previous "xcr0 == 0" path was
> critical to Xen not exploding on non-xsave platforms.
> 
> This is straight up buggy - we shouldn't be registering xsave handlers
> on non-xsave platforms, but the calculation is also wrong (in the safe
> directly at least) when we use compressed formats.  Yet another
> unexpected surprise for the todo list.

I don't view this as buggy at all - it was an implementation choice.
Perhaps not the best one, but still correct afaict. Then again I'm
afraid I don't understand "in the safe directly at least", so I may
well be overlooking something. Will wait for your updated patch ...

Jan


  reply	other threads:[~2021-05-04 12:08 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-03 15:39 [PATCH 0/5] x86/xstate: Fixes to size calculations Andrew Cooper
2021-05-03 15:39 ` [PATCH 1/5] x86/xstate: Elide redundant writes in set_xcr0() Andrew Cooper
2021-05-04 11:51   ` Jan Beulich
2021-05-03 15:39 ` [PATCH 2/5] x86/xstate: Rename _xstate_ctxt_size() to hw_uncompressed_size() Andrew Cooper
2021-05-04 11:53   ` Jan Beulich
2021-05-03 15:39 ` [PATCH 3/5] x86/xstate: Rework xstate_ctxt_size() as xstate_uncompressed_size() Andrew Cooper
2021-05-03 18:17   ` Andrew Cooper
2021-05-04 12:08     ` Jan Beulich [this message]
2021-05-04 12:15       ` Andrew Cooper
2021-05-04 12:20   ` Jan Beulich
2021-05-04 12:22     ` Andrew Cooper
2021-05-04 12:45       ` Jan Beulich
2021-05-03 15:39 ` [PATCH 4/5] x86/cpuid: Simplify recalculate_xstate() Andrew Cooper
2021-05-04 12:43   ` Jan Beulich
2021-05-04 13:58     ` Andrew Cooper
2021-05-05  8:19       ` Jan Beulich
2021-05-05 14:53         ` Andrew Cooper
2021-05-05 15:14           ` Jan Beulich
2021-05-03 15:39 ` [PATCH 5/5] x86/cpuid: Fix handling of xsave dynamic leaves Andrew Cooper
2021-05-04 12:56   ` Jan Beulich
2021-05-04 14:17     ` Andrew Cooper
2021-05-05  8:33       ` Jan Beulich
2021-05-05 16:59         ` Andrew Cooper
2021-05-06  6:17           ` Jan Beulich

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