[PATCH 13/18] ntdll: Use syscall frame for YMM context in x86_64 NtGetContextThread.

[Paul Gofman]

On 1/22/21 20:51, Jacek Caban wrote:
> On 22.01.2021 18:44, Paul Gofman wrote:
>>> What seems to be more interesting is xsaveopt, which I think could
>>> make a difference. That would, however, need xsave are to be at
>>> constant address. I've been thinking about storing it next to TEB, but
>>> we can't do that as long as winsock is called on signal stack, so I
>>> left experimenting with it for the future.
>>>
>> xsavec also performs an optimization (doesn't save the xstate in initial
>> state),
>
>
> As I said, xsave already does that optimization, there is no need for
> xsavec for that.
>
Are you sure? As I read the docs, there is no mention of xsave avoiding
any saves (e. g., [1], [2]), and I probably even tested that (well, not
100% sure now_ xsave zeros the data while xsavec (expectedly) does not.:

>From 2 (savec):

TO_BE_SAVED ← RFBM AND XINUSE;

>From 1:

Only "RFBM ← XCR0 AND EDX:EAX;" is used as a flag controlling whether
save is performed or not.


1. https://www.felixcloutier.com/x86/xsave

2. https://www.felixcloutier.com/x86/xsavec

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