Re: [PATCH 1/4] x86: extend update_intpte() to support atomic get-and-update

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 13/08/2026 1:01 pm, Jan Beulich wrote:
> On 27.07.2026 17:06, Kevin Lampis wrote:
>> --- a/xen/arch/x86/pv/mm.h
>> +++ b/xen/arch/x86/pv/mm.h
>> @@ -66,13 +66,14 @@ static inline intpte_t paging_cmpxchg_guest_entry(
>>   * How to write an entry to the guest pagetables.
>>   * Returns false for failure (pointer not valid), true for success.
>>   */
>> -static inline bool update_intpte(intpte_t *p, intpte_t old, intpte_t new,
>> -                                 mfn_t mfn, struct vcpu *v, bool preserve_ad)
>> +static inline bool update_intpte(intpte_t *p, intpte_t *old, intpte_t new,
>> +                                 mfn_t mfn, struct vcpu *v, bool preserve_ad,
>> +                                 bool use_cmpxchg)
> No 2nd boolean parameter, please. (use_cmpxchg also doesn't look to be an
> overly good name; "swap" maybe?)

This is half of a patch that's been in the XenServer queue for decades
for other purposes.  (TLB-flush avoidance based on A/D being clear, for
which you must use some form of atomic, but it relies on dom0 being
trusted not to clear the A/D bits in isolation.)

I agree that we don't want more booleans.  Your flags proposal looks
like the right way to go.

XenServer's pre-existing usecase could get away with XCHG.  I think it
was wired into CMPXCHG simply because that already existed.  This new
usecase probably wants to be XCHG too.  I don't think "please preserve
AD while swapping X for Y and also tell the the old value you found"
makes much sense at the hypercall level at least.

Furthermore, now that the return value is unused, we could return the
actual old value to anyone who cares, which avoids turning the input
"old" value into a pointer.

~Andrew

P.S. looking at the preserve_ad logic, I think it ought to be tightened
to only permit A/D becoming set, because that's the only direction that
hardware will move the bits.  A/D becoming clear is a race against
something which is not the pagewalker.