Re: [PATCH] KVM: arm64: ptdump: Flush the last region

[Mark Rutland]

On Mon, Jul 20, 2026 at 11:58:44AM +0100, Wei-Lin Chang wrote:
> On Mon, Jul 20, 2026 at 09:35:01AM +0100, Mark Rutland wrote:
> > On Sat, Jul 18, 2026 at 12:12:33AM +0100, Wei-Lin Chang wrote:
> > > @@ -155,11 +155,13 @@ static int kvm_ptdump_guest_show(struct seq_file *m, void *unused)
> > >  		.seq		= m,
> > >  	};
> > >  
> > > -	write_lock(&kvm->mmu_lock);
> > > +	guard(write_lock)(&kvm->mmu_lock);
> > >  	ret = kvm_pgtable_walk(mmu->pgt, 0, BIT(mmu->pgt->ia_bits), &walker);
> > > -	write_unlock(&kvm->mmu_lock);
> > > +	if (ret)
> > > +		return ret;
> > > +	note_page(&st->parser_state.ptdump, BIT(mmu->pgt->ia_bits), -1, 0);
> > 
> > This can be:
> > 
> > 	note_page_flush(&st->parser_state.ptdump);
> > 
> > The level change alone should trigger the dump, so the address doesn't
> > need to be at the end of the guest IPA space.
> > 
> > Importantly, note_page_flush() will pass 0 as the address, which won't
> > trigger the checks you try to suppress below.
> > 
> > Please use note_page_flush() here, and drop the changes to
> > arch/arm64/mm/ptdump.c.
> 
> Sorry, I shouldn't have omitted this information, but I did try
> note_page_flush(). And it gives something like this in the last row:
> 
> 0x00000000ffc00000-0x0000000000000000   17592186040324M 2   R W px ux  AF BLK
> 
> The astronomical size is from (addr - st->start_address). As IA bits for
> stage-2 are not close to 64, we'll have large sizes for the last row.

Ok. That's a bug in the current implementation of note_page_flush(),
then. The *intent* is that note_page_flush() is used to terminate
output, and we should mak it work.

Do we need to pass additional information, or do we have the necessary
values in (or accessible from) struct ptdump_state?

> That's one of the reasons I chose to call note_page() with
> BIT(pgtable->ia_bits) as addr, to end the ptdump at the end of the guest
> IPA space.
> 
> Additionally, the last row is combining two ranges:
> 
> 1. 0x00000000ffc00000-0x0000000100000000   4M       2    R W px ux  AF BLK
> 2. 0x0000000100000000-0x0000000000000000   BIG_SIZE -    (empty prot)
> 
> The attributes are wrong for the large range after
> BIT(pgtable->ia_bits). This is because before dumping the last row, the
> ptdump code is waiting to be notified of the end of the final region
> with all those {R, W, px, ux, AF, BLK} attributes. Using
> note_page_flush() essentially tells it the valid range ends at 1 << 64.
> So actually using note_page() with BIT(pgtable->ia_bits) is required for
> correctness.
> 
> The kernel ptdump is not affected by this (at least from my quick test):
> 
> 0xffffffffff6fe000-0xffffffffff800000        1032K PTE
> 0xffffffffff800000-0x0000000000000000           8M PMD

Yes, it's not affected because its final VA is 0xffffffffffffffff (i.e.
2^64i - 1), and so using 0 as the next address does the right thing
modulo 64 bits.

We use ptdump for non-kernel tables today (e.g. the EFI mm), so
presumably they suffer the same problem?

Mark.