Re: [PATCH v5 24/28] x86/fpu/xstate: Use per-task xstate mask for saving xstate in signal frame

[Andy Lutomirski]

On Mon, May 24, 2021, at 11:06 AM, Len Brown wrote:
> On Sun, May 23, 2021 at 11:15 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > If I'm reading this right, it means that tasks that have ever used AMX
> > get one format and tasks that haven't get another one.
> 
> No.  The format of the XSTATE on the signal stack is uncompressed XSAVE
> format for both AMX and non-AMX tasks, both before and after this patch.
> That is because XSAVE gets the format from XCR0.  It gets the fields
> to write from the run-time parameter.
> 
> So the change here allows a non-AMX task to skip writing data (zeros)
> to the AMX region of its XSTATE buffer.

I misread the patch. I still think this patch is useless.

> 
> The subsequent patch adds the further optimization of (manually) checking
> for INIT state for an AMX-task and also skip writing data (zeros) in that case.
> 
> We should have done this optimization for AVX-512, but instead we
> guaranteed writing zeros, which I think is a waste of both transfer time
> and cache footprint.

If no one depends on it, it’s not ABI.