Hi Julien,
> On 22 May 2024, at 14:25, Julien Grall <jul...@xen.org> wrote:
>
>> diff --git a/xen/arch/arm/mmu/p2m.c b/xen/arch/arm/mmu/p2m.c
>> index 41fcca011cf4..b496266deef6 100644
>> --- a/xen/arch/arm/mmu/p2m.c
>> +++ b/xen/arch/arm/mmu/p2m.c
>> @@ -753,17 +753,9 @@ static int p2m_mem_access_radix_set(struct p2m_domain
>> *p2m, gfn_t gfn,
>> return rc;
>> }
>> -/*
>> - * Put any references on the single 4K page referenced by pte.
>> - * TODO: Handle superpages, for now we only take special references for leaf
>> - * pages (specifically foreign ones, which can't be super mapped today).
>> - */
>> -static void p2m_put_l3_page(const lpae_t pte)
>> +/* Put any references on the single 4K page referenced by mfn. */
>> +static void p2m_put_l3_page(mfn_t mfn, p2m_type_t type)
>> {
>> - mfn_t mfn = lpae_get_mfn(pte);
>> -
>> - ASSERT(p2m_is_valid(pte));
>> -
>> /*
>> * TODO: Handle other p2m types
>> *
>> @@ -771,16 +763,43 @@ static void p2m_put_l3_page(const lpae_t pte)
>> * flush the TLBs if the page is reallocated before the end of
>> * this loop.
>> */
>> - if ( p2m_is_foreign(pte.p2m.type) )
>> + if ( p2m_is_foreign(type) )
>> {
>> ASSERT(mfn_valid(mfn));
>> put_page(mfn_to_page(mfn));
>> }
>> /* Detect the xenheap page and mark the stored GFN as invalid. */
>> - else if ( p2m_is_ram(pte.p2m.type) && is_xen_heap_mfn(mfn) )
>> + else if ( p2m_is_ram(type) && is_xen_heap_mfn(mfn) )
>> page_set_xenheap_gfn(mfn_to_page(mfn), INVALID_GFN);
>> }
>
> All the pages within a 2MB mapping should be the same type. So...
>
>> +/* Put any references on the superpage referenced by mfn. */
>> +static void p2m_put_l2_superpage(mfn_t mfn, p2m_type_t type)
>> +{
>> + unsigned int i;
>> +
>> + for ( i = 0; i < XEN_PT_LPAE_ENTRIES; i++ )
>> + {
>> + p2m_put_l3_page(mfn, type);
>> +
>> + mfn = mfn_add(mfn, 1);
>> + }
>
> ... this solution is a bit wasteful as we will now call p2m_put_l3_page() 512
> times even though there is nothing to do.
>
> So instead can we move the checks outside to optimize the path a bit?
You mean this?
diff --git a/xen/arch/arm/mmu/p2m.c b/xen/arch/arm/mmu/p2m.c
index b496266deef6..d40cddda48f3 100644
--- a/xen/arch/arm/mmu/p2m.c
+++ b/xen/arch/arm/mmu/p2m.c
@@ -794,7 +794,8 @@ static void p2m_put_page(const lpae_t pte, unsigned int
level)
ASSERT(p2m_is_valid(pte));
/* We have a second level 2M superpage */
- if ( p2m_is_superpage(pte, level) && (level == 2) )
+ if ( p2m_is_superpage(pte, level) && (level == 2) &&
+ p2m_is_foreign(pte.p2m.type) )
return p2m_put_l2_superpage(mfn, pte.p2m.type);
else if ( level == 3 )
return p2m_put_l3_page(mfn, pte.p2m.type);
> Otherwise...
>
>> +}
>> +
>> +/* Put any references on the page referenced by pte. */
>> +static void p2m_put_page(const lpae_t pte, unsigned int level)
>> +{
>> + mfn_t mfn = lpae_get_mfn(pte);
>> +
>> + ASSERT(p2m_is_valid(pte));
>> +
>> + /* We have a second level 2M superpage */
>> + if ( p2m_is_superpage(pte, level) && (level == 2) )
>> + return p2m_put_l2_superpage(mfn, pte.p2m.type);
>> + else if ( level == 3 )
>> + return p2m_put_l3_page(mfn, pte.p2m.type);
>> +}
>> +
>> /* Free lpae sub-tree behind an entry */
>> static void p2m_free_entry(struct p2m_domain *p2m,
>> lpae_t entry, unsigned int level)
>> @@ -809,9 +828,16 @@ static void p2m_free_entry(struct p2m_domain *p2m,
>> #endif
>> p2m->stats.mappings[level]--;
>> - /* Nothing to do if the entry is a super-page. */
>> - if ( level == 3 )
>> - p2m_put_l3_page(entry);
>> + /*
>> + * TODO: Currently we don't handle 1GB super-page, Xen is not
>> + * preemptible and therefore some work is needed to handle such
>> + * superpages, for which at some point Xen might end up freeing
>> memory
>> + * and therefore for such a big mapping it could end up in a very
>> long
>> + * operation.
>> + */
>> + if ( level >= 2 )
>> + p2m_put_page(entry, level);
>> +
>> return;
>> }
>> @@ -1558,9 +1584,12 @@ int relinquish_p2m_mapping(struct domain *d)
>> count++;
>> /*
>> - * Arbitrarily preempt every 512 iterations.
>> + * Arbitrarily preempt every 512 iterations or when type is foreign
>> + * mapping and the order is above 9 (2MB).
>> */
>> - if ( !(count % 512) && hypercall_preempt_check() )
>> + if ( (!(count % 512) ||
>> + (p2m_is_foreign(t) && (order > XEN_PT_LEVEL_ORDER(2)))) &&
>
> ... we would need to preempt for every 2MB rather than just for the
> p2m_is_foreign().
Ok otherwise you are suggesting that if we don’t go for the solution above we
drop p2m_is_foreign(t) from
the condition here, am I right?
>
> BTW, p2m_put_l3_page() has also another case. Should we consider to handle
> preemption for it too?
You mean checking for 512 iterations, or foreign mapping when order is > 9, or
p2m_is_ram(type) && is_xen_heap_mfn(mfn) ?
Just want to be sure I fully understand your comments here.
Cheers,
Luca
