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Re: [PATCH 3/5] x86/xstate: Rework xstate_ctxt_size() as xstate_uncompressed_size()
- To: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 3 May 2021 19:17:48 +0100
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- Cc: Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Mon, 03 May 2021 18:18:11 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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@xxxxxxxxxx>
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.
~Andrew
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