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

[David Hildenbrand <david@redhat.com>]

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