Re: [PATCH resend v2 1/5] tpm: mark correct memory region range dirty when clearing RAM

[David Hildenbrand]

On 24.07.21 00:35, Peter Xu wrote:
> On Fri, Jul 23, 2021 at 09:15:43PM +0200, David Hildenbrand wrote:
> > On 23.07.21 16:52, Peter Xu wrote:
> > > On Tue, Jul 20, 2021 at 03:03:00PM +0200, David Hildenbrand wrote:
> > > > @@ -30,11 +30,13 @@ void tpm_ppi_reset(TPMPPI *tpmppi)
> > > >            guest_phys_blocks_init(&guest_phys_blocks);
> > > >            guest_phys_blocks_append(&guest_phys_blocks);
> > > >            QTAILQ_FOREACH(block, &guest_phys_blocks.head, next) {
> > > > +            ram_addr_t mr_start = memory_region_get_ram_addr(block->mr);
> > > > +
> > > >                trace_tpm_ppi_memset(block->host_addr,
> > > >                                     block->target_end - block->target_start);
> > > >                memset(block->host_addr, 0,
> > > >                       block->target_end - block->target_start);
> > > > -            memory_region_set_dirty(block->mr, 0,
> > > > +            memory_region_set_dirty(block->mr, block->target_start - mr_start,
> > > >                                        block->target_end - block->target_start);
> > >
> > > target_start should falls in gpa range, while mr_start is ram_addr_t.  I am not
> > > sure whether this is right..
> >
> > When I wrote that code I was under the impression that
> > memory_region_get_ram_addr() would give the GPA where the memory region
> > starts, but ... that's not correct as you point out. "offset" confusion :)
> >
> > > Neither do I know how to get correct mr offset with the existing info we've got
> > > from GuestPhysBlock.  Maybe we need to teach guest_phys_blocks_region_add() to
> > > also record section->offset_within_region?
> >
> > We might actually want offset_within_address_space + offset_within_region,
> > so we can calculate the GPA difference to see where inside the ramblock we
> > end up.
>
> I still think offset_within_region is exactly what we want to fill in here, but
> you can do a double check.

I remember when I first looked into that months ago I wanted to avoid
extending GuestPhysBlock. The commit message actually tells us what to do,
and where my optimization went wrong :)

"We might not start at the beginning of the memory region. We could also
 calculate via the difference in the host address; however,
 memory_region_set_dirty() also relies on memory_region_get_ram_addr()
 internally, so let's just use that."

So, avoiding the optimization, we'd be left with:


diff --git a/hw/tpm/tpm_ppi.c b/hw/tpm/tpm_ppi.c
index 362edcc5c9..fab49524d7 100644
--- a/hw/tpm/tpm_ppi.c
+++ b/hw/tpm/tpm_ppi.c
@@ -30,11 +30,14 @@ void tpm_ppi_reset(TPMPPI *tpmppi)
         guest_phys_blocks_init(&guest_phys_blocks);
         guest_phys_blocks_append(&guest_phys_blocks);
         QTAILQ_FOREACH(block, &guest_phys_blocks.head, next) {
+            hwaddr mr_offs = (uint8_t *) memory_region_get_ram_ptr(block->mr) -
+                             block->host_addr;
+
             trace_tpm_ppi_memset(block->host_addr,
                                  block->target_end - block->target_start);
             memset(block->host_addr, 0,
                    block->target_end - block->target_start);
-            memory_region_set_dirty(block->mr, 0,
+            memory_region_set_dirty(block->mr, mr_offs,
                                     block->target_end - block->target_start);
         }
         guest_phys_blocks_free(&guest_phys_blocks);


That should make more sense :)


--
Thanks,

David / dhildenb