Re: [XEN][PATCH v4] xen/x86: guest_access: optimize raw_x_guest() for PV and HVM combinations

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.11.2025 15:01, 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 move arch/x86/usercopy.c into arch/x86/pv/usercopy.c to use it only
> with PV=y.
> 
> The measured (bloat-o-meter) improvement for (PV=n and HVM=y) case is:
>   add/remove: 3/8 grow/shrink: 3/89 up/down: 1018/-12087 (-11069)
>   Total: Before=1937280, After=1926211, chg -0.57%
> 
> [teddy.astie@xxxxxxxxxx: Suggested to use static inline functions vs
> macro combinations]
> Suggested-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
> Signed-off-by: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
> Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>

I would guess that this R-b would have needed dropping, ...

> ---
> changes in v4:
> - move usercopy.c into arch/x86/pv/
> - rework to always dynamically check for HVM vcpu(domain) by using 
> is_hvm_vcpu()
>   as requested by Jan Beulich

... with at least the latter of these two changes.

> --- a/xen/arch/x86/pv/Makefile
> +++ b/xen/arch/x86/pv/Makefile
> @@ -14,6 +14,10 @@ obj-y += ro-page-fault.o
>  obj-$(CONFIG_PV_SHIM) += shim.o
>  obj-$(CONFIG_TRACEBUFFER) += trace.o
>  obj-y += traps.o
> +obj-$(CONFIG_PV) += usercopy.o

Just obj-y with the movement.

However, is the movement (and was the adding of $(CONFIG_PV) in the earlier
version) actually correct? The file also produces copy_{from,to}_unsafe_ll(),
which aren't PV-specific. This may be only a latent issue right now, as we
have only a single use site of copy_from_unsafe(), but those functions need
to remain available. (We may want to arrange for them to be removed when
linking, as long as they're not referenced. But that's a separate topic.)

Jan