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Re: [XEN][PATCH v3] xen/x86: guest_access: optimize raw_x_guest() for PV and HVM combinations
- To: Grygorii Strashko <grygorii_strashko@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jason Andryuk <jason.andryuk@xxxxxxx>
- Date: Fri, 7 Nov 2025 13:44:04 -0500
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>
- Delivery-date: Fri, 07 Nov 2025 18:44:24 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 2025-11-07 13:17, 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%
[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>
Thanks,
Jason
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