Re: [PATCH v2] x86/mm: limit deferred TLB flushing to PV domain support

[Jason Andryuk <jason.andryuk@amd.com>]

On 2026-09-09 12:28, Roger Pau Monné wrote:
> On Wed, Sep 09, 2026 at 04:46:01PM +0200, Jan Beulich wrote:
>> On 09.09.2026 16:05, Roger Pau Monne wrote:
>>> --- a/xen/common/page_alloc.c
>>> +++ b/xen/common/page_alloc.c
>>> @@ -1538,8 +1538,11 @@ static bool mark_page_free(struct page_info *pg, mfn_t mfn)
>>>           BUG();
>>>       }
>>>   
>>> -    /* If a page has no owner it will need no safety TLB flush. */
>>> -    pg->u.free.need_tlbflush = (page_get_owner(pg) != NULL);
>>> +    /*
>>> +     * If a page has no owner and there's no PV domain support it will need no
>>> +     * safety TLB flush, there can be no stale TLB entries.
>>> +     */
>>> +    pg->u.free.need_tlbflush = IS_ENABLED(CONFIG_PV) && page_get_owner(pg);
>>>       if ( pg->u.free.need_tlbflush )
>>>           page_set_tlbflush_timestamp(pg);
>>
>> I'm okay with the code change now, but the comment is still concerning me.
>> All by itself there is no reason why stale TLB entries couldn't also exist
>> for HVM guests. It's just that (a) only the host TLBs are flushed by
>> filtered_flush_tlb_mask() and (b) flushes of guest TLBs occur when pages
>> are removed from their P2Ms (aiui; hopefully true also for Arm). IOW what
>> the comment says looks to be correct, just that it leaves too much to be
>> figured out by the reader. At the very least I'd suggest "..., there can
>> be no stale (host) TLB entries." Thoughts?
> 
> Hm, I find adding "(host)" to also be slightly confusing, as I would
> usually associate host TLB with Xen context TLB state.  Which is also
> made more confusing by how PV guests share the page-tables with Xen.
> 
> "If a page has no owner and there's no PV domain support it will need
> no safety TLB flush.  PV domains are the only domain types that can
> keep stale entries on the TLB, as they have (limited) control over the
> host MMU and when flushes are performed"
I find "will need no" a little awkward.  Maybe:

"If a page has no owner and there's no PV domain support it does not 
need a safety TLB flush."

or:

"If a page has no owner and there's no PV domain support, then a safety 
TLB flush is not needed."

Regards,
Jason