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Re: [PATCH v2 16/16] x86/fsgsbase: Improve code generation in read_registers()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 22 Aug 2025 08:32:27 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Fri, 22 Aug 2025 06:32:43 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21.08.2025 22:52, Andrew Cooper wrote:
> On 19/08/2025 2:19 pm, Jan Beulich wrote:
>> On 15.08.2025 22:41, Andrew Cooper wrote:
>>> It turns out that using the higher level helpers adjacent like this leads to
>>> terrible code generation. Due to -fno-strict-alising, the store into
>>> state->
>>> invalidates the read_cr4() address calculation (which is really
>>> cpu_info->cr4
>>> under the hood), meaning that it can't be hoisted.
>>>
>>> As a result we get "locate the top of stack block, get cr4, and see if
>>> FSGSBASE is set" repeated 3 times, and an unreasoanble number of basic
>>> blocks.
>>>
>>> Hoist the calculation manually, which results in two basic blocks.
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Otoh the function here isn't really performance or size critical. I'm
>> undecided
>> whether the undesirable open-coding or the bad code gen are the lesser evil.
>
> This function no, but every other place touching FS and GS is
> performance critical. They're all messy to start with, and get worse
> under FRED.
Is there any (further) bad effect to the function here by the time all of the
FRED bits are in?
Jan
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