Re: [RFC PATCH] hw/virtio/vhost: re-factor vhost-section and allow DIRTY_MEMORY_CODE

["Alex Bennée" <alex.bennee@linaro.org>]

Michael S. Tsirkin <mst@redhat.com> writes:

> On Thu, Jun 04, 2020 at 12:13:23PM +0100, Alex Bennée wrote:
>> The purpose of vhost_section is to identify RAM regions that need to
>> be made available to a vhost client. However when running under TCG
>> all RAM sections have DIRTY_MEMORY_CODE set which leads to problems
>> down the line. The original comment implies VGA regions are a problem
>> but doesn't explain why vhost has a problem with it.
>> 
>> Re-factor the code so:
>> 
>>   - steps are clearer to follow
>>   - reason for rejection is recorded in the trace point
>>   - we allow DIRTY_MEMORY_CODE when TCG is enabled
>> 
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> Cc: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>>  hw/virtio/vhost.c | 46 ++++++++++++++++++++++++++++++++--------------
>>  1 file changed, 32 insertions(+), 14 deletions(-)
>> 
>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>> index aff98a0ede5..f81fc87e74c 100644
>> --- a/hw/virtio/vhost.c
>> +++ b/hw/virtio/vhost.c
>> @@ -27,6 +27,7 @@
>>  #include "migration/blocker.h"
>>  #include "migration/qemu-file-types.h"
>>  #include "sysemu/dma.h"
>> +#include "sysemu/tcg.h"
>>  #include "trace.h"
>>  
>>  /* enabled until disconnected backend stabilizes */
>> @@ -403,26 +404,43 @@ static int vhost_verify_ring_mappings(struct vhost_dev *dev,
>>      return r;
>>  }
>>  
>> +/*
>> + * vhost_section: identify sections needed for vhost access
>> + *
>> + * We only care about RAM sections here (where virtqueue can live). If
>> + * we find one we still allow the backend to potentially filter it out
>> + * of our list.
>> + */
>>  static bool vhost_section(struct vhost_dev *dev, MemoryRegionSection *section)
>>  {
>> -    bool result;
>> -    bool log_dirty = memory_region_get_dirty_log_mask(section->mr) &
>> -                     ~(1 << DIRTY_MEMORY_MIGRATION);
>> -    result = memory_region_is_ram(section->mr) &&
>> -        !memory_region_is_rom(section->mr);
>> -
>> -    /* Vhost doesn't handle any block which is doing dirty-tracking other
>> -     * than migration; this typically fires on VGA areas.
>> -     */
>> -    result &= !log_dirty;
>> +    enum { OK = 0, NOT_RAM, DIRTY, FILTERED } result = NOT_RAM;
>
> I'm not sure what does this enum buy us as compared to bool.

The only real point of the enum is to give a little more detailed
information to the trace point to expose why a section wasn't included.
In a previous iteration I just had the tracepoint at the bottom before a
return true where all other legs had returned false. We could switch to
just having the tracepoint hit for explicit inclusions?

> Also why force OK to 0?

Personal preference where 0 indicates success and !0 indicates failure
of various kinds. Again we can drop if we don't want the information in
the tracepoint.

> And I prefer an explicit "else result = NOT_RAM" below
> instead of initializing it here.

Ok.

>
>> +
>> +    if (memory_region_is_ram(section->mr) && !memory_region_is_rom(section->mr)) {
>> +        uint8_t dirty_mask = memory_region_get_dirty_log_mask(section->mr);
>> +        uint8_t handled_dirty;
>>  
>> -    if (result && dev->vhost_ops->vhost_backend_mem_section_filter) {
>> -        result &=
>> -            dev->vhost_ops->vhost_backend_mem_section_filter(dev, section);
>> +        /*
>> +         * Vhost doesn't handle any block which is doing dirty-tracking other
>> +         * than migration; this typically fires on VGA areas. However
>> +         * for TCG we also do dirty code page tracking which shouldn't
>> +         * get in the way.
>> +         */
>> +        handled_dirty = (1 << DIRTY_MEMORY_MIGRATION);
>> +        if (tcg_enabled()) {
>> +            handled_dirty |= (1 << DIRTY_MEMORY_CODE);
>> +        }
>
> So DIRTY_MEMORY_CODE is only set by TCG right? Thus I'm guessing
> we can just allow this unconditionally.

Which actually makes the test:

  if (dirty_mask & DIRTY_MEMORY_VGA) {
     .. fail ..
  }

which is more in line with the comment although wouldn't fail if we
added additional DIRTY_MEMORY flags. This leads to the question what
exactly is it about DIRTY tracking that vhost doesn't like. Is it really
only avoiding having virtqueue in video RAM? Does this ever actually
happen?

I assume boards with unified memory models where video ram is shared
with system ram just end up partitioning the memory regions?

-- 
Alex Bennée