Re: [PATCH 2/2] x86/fpu: Initialise FTW as well as FCW in xstate_alloc_save_area()

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 3/26/26 7:04 PM, Andrew Cooper wrote:
> xstate_alloc_save_area() configures FCW and MXCSR to #RESET values but misses
> FTW.  Fixing this means that the backing memory always has an architecturally
> correct value.
>
> Adjust the comment to state that it's the #RESET values which we care about.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>
> I don't understand what the rest of the comment is trying to say, so have left
> it alone.  There's still a lot of cleanup to be done to merge i387 and xstate.
> ---
>   xen/arch/x86/xstate.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
> index e990abc9d18c..747df0b2e9a9 100644
> --- a/xen/arch/x86/xstate.c
> +++ b/xen/arch/x86/xstate.c
> @@ -550,11 +550,12 @@ int xstate_alloc_save_area(struct vcpu *v)
>           return -ENOMEM;
>   
>       /*
> -     * Set the memory image to default values, but don't force the context
> +     * Set the memory image to #RESET values, but don't force the context
>        * to be loaded from memory (i.e. keep save_area->xsave_hdr.xstate_bv
>        * clear).
>        */
>       save_area->fpu_sse.fcw = FCW_DEFAULT;
> +    save_area->fpu_sse.ftw = FXSAVE_FTW_RESET;
>       save_area->fpu_sse.mxcsr = MXCSR_DEFAULT;
>   
>       v->arch.xsave_area = save_area;
Is this comment correct given that it is initializing FCW to FCW_DEFAULT
which is different from FCW_RESET?

As they seem to be mostly doing the same thing, could we call
vcpu_reset_fpu() here instead?

Ross