Re: [PATCH v4] KVM: arm64: Record whether pKVM stage 2 mapping is cacheable

On Thu, Jul 02, 2026 at 03:52:14PM +0100, Bradley Morgan wrote:

On July 2, 2026 12:18:58 PM GMT+01:00, Leonardo Bras [off-list ref]
wrote:

quoted

On Thu, Jul 02, 2026 at 09:59:23AM +0100, Marc Zyngier wrote:

quoted

+ Vincent, Leo

On Wed, 01 Jul 2026 20:24:28 +0100,
Bradley Morgan [off-list ref] wrote:

quoted

pKVM keeps its own mapping list for stage 2 operations. Its flush path
uses that list directly, so it lost the PTE attribute check done by

the

quoted

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generic stage 2 walker.

Record whether a mapping is cacheable and skip cache maintenance for
mappings that are not cacheable.

Fixes: e912efed485a ("KVM: arm64: Introduce the EL1 pKVM MMU")
Signed-off-by: Bradley Morgan <redacted>
---
Changes since V3:
- addressed some review :)

This isn't a change log. If you want to be taken seriously, I'd
suggest you start by following the process. You are otherwise wasting
people's time. Again.

Agree... the process has a reason: the changelog here helps the reviewer 
having an overview of what actually changed between versions, and that 
makes reviewing much easier, and costs very little to the contributor.

Also, the 1 week waiting time is really important, as mentioned before, as
it allows more reviewers and maintainers to give feedback: different 
people, from different companies and lifestyles have different schedules 
for dev/rev, but in general it happens weekly. So waiting for a week is 
really recommended, as it tends to avoid people re-reading 2+ versions of 
the same patchset, and allows revs time to discuss the suggestions in the 
same thread.

(I know the waiting can be really frustating, and that sending a vN+1 fast
seems to show that you are interested in it, but it really does not help)

Well, it's fair, let me give you reasoning on why I do quick Rerolls.

So, if I do something wrong, e.g: checkpatch, I don't want to wait a week,
because I would get grilled for a said checkpatch failure.

Fair, but people would not avoid reviewing your new version due to a 
checkpath/etc issue. It's common to just report/ignore that and go on with 
the code that matters. 

Also, checkpath is a good example of what you can run in your code before 
sending, so it does not become a respin.

In this case, I was just excited /shrug.

It's okay. Been there as well :)

quoted

quoted

You also failed to Cc people who have provided feedback on previous
versions. That's not right.

(Bradley: usually you want as many people as possible to review your
stuff, 
so CC'ing previous reviewers is actually good for you)

Vincent not being CCed was dumb of me, no idea how I didn't get him on get
maintainers.

For you, I didn't want to annoy you with another patch, since maintainers
tend to be stressed and annoyed. And I didn't wanna add on to the pain.

Don't worry on this. Previous reviewers can just ignore the e-mail if 
they don't find it interesting anymore.

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 arch/arm64/kvm/pkvm.c | 51

++++++++++++++++++++++++++++++++++---------

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 1 file changed, 41 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c
index 053e4f733e4b..6d1cad890c7e 100644
--- a/arch/arm64/kvm/pkvm.c
+++ b/arch/arm64/kvm/pkvm.c

@@ -302,9 +302,32 @@ static u64 __pkvm_mapping_start(struct

pkvm_mapping *m)

quoted

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 	return m->gfn * PAGE_SIZE;
 }
 
+#define PKVM_MAPPING_NR_PAGES_MASK     GENMASK_ULL(47, 0)
+#define PKVM_MAPPING_NC                        BIT_ULL(48)
+
+static u64 pkvm_mapping_nr_pages(struct pkvm_mapping *m)
+{
+	return m->nr_pages & PKVM_MAPPING_NR_PAGES_MASK;
+}

No. You've been pointed to the correct data structure (an anonymous
structure containing bit fields). Please consider taking the review
comments into account.

(and if you do not agree with the suggestion, discuss it in the same 
thread. Although as Marc shows below, it becames much simpler like that)

Thanks! 
Leo

I'll test it. If it's good. I'll do 

Suggested-by? Or co-developed by?

I would say none. It's just a change suggestion to your patch, made by a 
reviewer. 

IIRC 'Suggested-by' is used if someone suggested the idea for the patch, 
such as "ah, you should do this thing before", etc. It's both for giving 
credit and showing people the idea for that was not yours, for multiple 
reasons.

I'll hold the patch on for a week.

Good! 
If you get too excited, I suggest reading other people's patches. Everybody 
makes mistakes, and the solutions / suggestions are really interesting :)

Thanks you lot for taking my bull crap.

It's not bad, the process just goes like that for everybody.
It's just slower than people expect :)

We get slightly better over time, but still make mistakes.
(And try our best not to make the same mistakes again :)

Thanks for contributing!
Leo

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This would avoid most of the churn in this patch, and make it easy to
backport. Something like the untested hack below.

	M.

diff --git a/arch/arm64/include/asm/kvm_pkvm.h

b/arch/arm64/include/asm/kvm_pkvm.h

quoted

index 74fedd9c5ff02..cdddc9e3a11f5 100644

--- a/arch/arm64/include/asm/kvm_pkvm.h
+++ b/arch/arm64/include/asm/kvm_pkvm.h

@@ -195,7 +195,10 @@ struct pkvm_mapping {
 	struct rb_node node;
 	u64 gfn;
 	u64 pfn;
-	u64 nr_pages;
+	struct {
+		unsigned long nr_pages:48;
+		unsigned int  nc:1;
+	};
 	u64 __subtree_last;	/* Internal member for interval tree */
 };
 

diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c
index 428723b1b0f5c..5932b93bded58 100644
--- a/arch/arm64/kvm/pkvm.c
+++ b/arch/arm64/kvm/pkvm.c

@@ -369,7 +369,7 @@ static int __pkvm_pgtable_stage2_unshare(struct

kvm_pgtable *pgt, u64 start, u64

quoted

 
 	for_each_mapping_in_range_safe(pgt, start, end, mapping) {
 		ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_guest, handle, mapping->gfn,
-					mapping->nr_pages);
+					(u64)mapping->nr_pages);
 		if (WARN_ON(ret))
 			return ret;
 		pkvm_mapping_remove(mapping, &pgt->pkvm_mappings);

@@ -473,6 +473,7 @@ int pkvm_pgtable_stage2_map(struct kvm_pgtable *pgt,

u64 addr, u64 size,

quoted

 	mapping->gfn = gfn;
 	mapping->pfn = pfn;
 	mapping->nr_pages = size / PAGE_SIZE;
+	mapping->nc = !!(prot & (KVM_PGTABLE_PROT_DEVICE | KVM_PGTABLE_PROT_NORMAL_NC));
 	pkvm_mapping_insert(mapping, &pgt->pkvm_mappings);
 
 	return ret;

@@ -503,7 +504,7 @@ int pkvm_pgtable_stage2_wrprotect(struct kvm_pgtable

*pgt, u64 addr, u64 size)

quoted

 	lockdep_assert_held(&kvm->mmu_lock);
 	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping) {
 		ret = kvm_call_hyp_nvhe(__pkvm_host_wrprotect_guest, handle, mapping->gfn,
-					mapping->nr_pages);
+					(u64)mapping->nr_pages);
 		if (WARN_ON(ret))
 			break;
 	}

@@ -517,10 +518,11 @@ int pkvm_pgtable_stage2_flush(struct kvm_pgtable

*pgt, u64 addr, u64 size)

quoted

 	struct pkvm_mapping *mapping;
 
 	lockdep_assert_held(&kvm->mmu_lock);
-	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping)
-		__clean_dcache_guest_page(pfn_to_kaddr(mapping->pfn),
-					  PAGE_SIZE * mapping->nr_pages);
-
+	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping) {
+		if (!mapping->nc)
+			__clean_dcache_guest_page(pfn_to_kaddr(mapping->pfn),
+						  PAGE_SIZE * mapping->nr_pages);
+	}
 	return 0;
 }
 

@@ -537,7 +539,7 @@ bool pkvm_pgtable_stage2_test_clear_young(struct

kvm_pgtable *pgt, u64 addr, u64

quoted

 	lockdep_assert_held(&kvm->mmu_lock);
 	for_each_mapping_in_range_safe(pgt, addr, addr + size, mapping)
 		young |= kvm_call_hyp_nvhe(__pkvm_host_test_clear_young_guest, handle, mapping->gfn,
-					   mapping->nr_pages, mkold);
+					   (u64)mapping->nr_pages, mkold);
 
 	return young;
 }

-- 
Without deviation from the norm, progress is not possible.

Thanks!