Re: [PATCH v7 38/41] x86/fpu: Add helper for initing features

[Borislav Petkov]

On Mon, Feb 27, 2023 at 02:29:54PM -0800, Rick Edgecombe wrote:
> Subject: Re: [PATCH v7 38/41] x86/fpu: Add helper for initing features

"initializing"

> 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

"its"

> 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.
> 
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 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 13a80521dd51..3ff80be0a441 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -934,6 +934,24 @@ 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)

Function name needs a verb.

> +{
> +	/*
> +	 * Do we even *have* xsave state?
> +	 */

That comment is superfluous.

> +	if (!boot_cpu_has(X86_FEATURE_XSAVE))

check_for_deprecated_apis: WARNING: arch/x86/kernel/fpu/xstate.c:942: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.

> +		return 1;
> +
> +	/*
> +	 * We should not ever be requesting features that we

Please use passive voice in your commit message: no "we" or "I", etc,
and describe your changes in imperative mood.

> +	 * 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.
> @@ -954,17 +972,7 @@ 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;
>  
>  	/*
> @@ -984,6 +992,34 @@ 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.

s/this function initializes/initialize/

> + *
> + * 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().

So users of get_xsave_addr() would have to know that they would need to
call init_xfeature()?

I think the better approach would be:

void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr, bool init)

and then that @init controls whether get_xsave_addr() should init the
buffer.

And then you don't have to have a bunch of small functions here and
there and know when to call what but get_xsave_addr() would simply DTRT.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette