Re: [PATCH v2] xen/x86: guest_access: optimize raw_x_guest() for PV and HVM combinations

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.11.2025 23:26, Grygorii Strashko wrote:
> From: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> 
> Xen uses below pattern for raw_x_guest() functions:
> 
> define raw_copy_to_guest(dst, src, len)        \
>     (is_hvm_vcpu(current) ?                     \
>      copy_to_user_hvm((dst), (src), (len)) :    \
>      copy_to_guest_pv(dst, src, len))
> 
> This pattern works depending on CONFIG_PV/CONFIG_HVM as:
> - PV=y and HVM=y
>   Proper guest access function is selected depending on domain type.
> - PV=y and HVM=n
>   Only PV domains are possible. is_hvm_domain/vcpu() will constify to "false"
>   and compiler will optimize code and skip HVM specific part.
> - PV=n and HVM=y
>   Only HVM domains are possible. is_hvm_domain/vcpu() will not be constified.
>   No PV specific code will be optimized by compiler.
> - PV=n and HVM=n
>   No guests should possible. The code will still follow PV path.
> 
> Rework raw_x_guest() code to use static inline functions which account for
> above PV/HVM possible configurations with main intention to optimize code
> for (PV=n and HVM=y) case.
> 
> For the case (PV=n and HVM=n) return "len" value indicating a failure (no
> guests should be possible in this case, which means no access to guest
> memory should ever happen).
> 
> Finally build arch/x86/usercopy.c only for PV=y.
> 
> The measured (bloat-o-meter) improvement for (PV=n and HVM=y) case is:
>   add/remove: 2/9 grow/shrink: 2/90 up/down: 1678/-32560 (-30882)
>   Total: Before=1937092, After=1906210, chg -1.59%
> 
> Signed-off-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> [teddy.astie@xxxxxxxxxx: Suggested to use static inline functions vs macro 
> combinations]
> Suggested-by: Teddy Astie <teddy.astie@xxxxxxxxxx>

Just one formal request for now: Please send patches To: the list, with 
individuals
on Cc: as necessary.

Jan