Re: [XEN][PATCH v3] xen/x86: guest_access: optimize raw_x_guest() for PV and HVM combinations

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.11.2025 18:52, Grygorii Strashko wrote:
> On 10.11.25 09:11, Jan Beulich wrote:
>> On 07.11.2025 19:17, Grygorii Strashko wrote:
>>> --- a/xen/arch/x86/include/asm/guest_access.h
>>> +++ b/xen/arch/x86/include/asm/guest_access.h
>>> @@ -13,26 +13,64 @@
>>>   #include <asm/hvm/guest_access.h>
>>>     /* Raw access functions: no type checking. */
>>> -#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))
>>> -#define raw_copy_from_guest(dst, src, len)      \
>>> -    (is_hvm_vcpu(current) ?                     \
>>> -     copy_from_user_hvm((dst), (src), (len)) :  \
>>> -     copy_from_guest_pv(dst, src, len))
>>> -#define raw_clear_guest(dst,  len)              \
>>> -    (is_hvm_vcpu(current) ?                     \
>>> -     clear_user_hvm((dst), (len)) :             \
>>> -     clear_guest_pv(dst, len))
>>> -#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))
>>> -#define __raw_copy_from_guest(dst, src, len)    \
>>> -    (is_hvm_vcpu(current) ?                     \
>>> -     copy_from_user_hvm((dst), (src), (len)) :  \
>>> -     __copy_from_guest_pv(dst, src, len))
>>> +static inline bool raw_use_hvm_access(const struct vcpu *v)
>>> +{
>>> +    return IS_ENABLED(CONFIG_HVM) && (!IS_ENABLED(CONFIG_PV) || 
>>> is_hvm_vcpu(v));
>>> +}
>>
>> Without a full audit (likely tedious and error prone) this still is a
>> behavioral change for some (likely unintended) use against a system domain
>> (likely the idle one): With HVM=y PV=n we'd suddenly use the HVM accessor
>> there. IOW imo the "system domains are implicitly PV" aspect wants
>> retaining, even if only "just in case". It's okay not to invoke the PV
>> accessor (but return "len" instead), but it's not okay to invoke the HVM
>> one.
> 
> This patch is subset of "constify is_hvm_domain() for PV=n case" attempts.
> 
> It was made under assumption that:
> "System domains do not have Guests running, so can't initiate hypecalls and
>  can not be users of copy_to/from_user() routines. There are no Guest and no 
> user memory".
> [IDLE, COW, IO, XEN]
> 
> If above assumption is correct - this patch was assumed safe.
> 
> if not - it all make no sense, probably.

I wouldn't go as far as saying that. It can be arranged to avid the corner
case I mentioned, I think.

Jan