Re: [OPTIONAL/RFC v2 36/39] x86/fpu: Add helper for initing features

[Chang S. Bae]

On 9/29/2022 3:29 PM, Rick Edgecombe wrote:
> If an xfeature is saved in a buffer, the xfeature's bit will be set in
> xsave->header.xfeatures. The CPU may opt to not save the xfeature if it
> is in it's init state. In this case the xfeature buffer address cannot
> be retrieved with get_xsave_addr().
>
> Future patches will need to handle the case of writing to an xfeature
> that may not be saved. So provide helpers to init an xfeature in an
> xsave buffer.
>
> This could of course be done directly by reaching into the xsave buffer,
> however this would not be robust against future changes to optimize the
> xsave buffer by compacting it. In that case the xsave buffer would need
> to be re-arranged as well. So the logic properly belongs encapsulated
> in a helper where the logic can be unified.
>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>
> ---
>
> v2:
>   - New patch
>
>   arch/x86/kernel/fpu/xstate.c | 58 +++++++++++++++++++++++++++++-------
>   arch/x86/kernel/fpu/xstate.h |  6 ++++
>   2 files changed, 53 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 9258fc1169cc..82cee1f2f0c8 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -942,6 +942,24 @@ static void *__raw_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
>   	return (void *)xsave + xfeature_get_offset(xcomp_bv, xfeature_nr);
>   }
>   
> +static int xsave_buffer_access_checks(int xfeature_nr)
> +{
> +	/*
> +	 * Do we even *have* xsave state?
> +	 */
> +	if (!boot_cpu_has(X86_FEATURE_XSAVE))
> +		return 1;
> +
> +	/*
> +	 * We should not ever be requesting features that we
> +	 * have not enabled.
> +	 */
> +	if (WARN_ON_ONCE(!xfeature_enabled(xfeature_nr)))
> +		return 1;
> +
> +	return 0;
> +}
> +
>   /*
>    * Given the xsave area and a state inside, this function returns the
>    * address of the state.
> @@ -962,17 +980,7 @@ static void *__raw_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
>    */
>   void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
>   {
> -	/*
> -	 * Do we even *have* xsave state?
> -	 */
> -	if (!boot_cpu_has(X86_FEATURE_XSAVE))
> -		return NULL;
> -
> -	/*
> -	 * We should not ever be requesting features that we
> -	 * have not enabled.
> -	 */
> -	if (WARN_ON_ONCE(!xfeature_enabled(xfeature_nr)))
> +	if (xsave_buffer_access_checks(xfeature_nr))
>   		return NULL;
>   
>   	/*
> @@ -992,6 +1000,34 @@ void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
>   	return __raw_xsave_addr(xsave, xfeature_nr);
>   }
>   
> +/*
> + * Given the xsave area and a state inside, this function
> + * initializes an xfeature in the buffer.

But, this function sets XSTATE_BV bits in the buffer. That does not 
*initialize* the state, right?

> + *
> + * get_xsave_addr() will return NULL if the feature bit is
> + * not present in the header. This function will make it so
> + * the xfeature buffer address is ready to be retrieved by
> + * get_xsave_addr().

Looks like this is used in the next patch to help ptracer().

We have the state copy function -- copy_uabi_to_xstate() that retrieves 
the address using __raw_xsave_addr() instead of get_xsave_addr(), copies 
the state, and then updates XSTATE_BV.

__raw_xsave_addr() also ensures whether the state is in the compacted 
format or not. I think you can use it.

Also, I'm curious about the reason why you want to update XSTATE_BV 
first with this new helper.

Overall, I'm not sure these new helpers are necessary.

Thanks,
Chang