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

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

Dr. David Alan Gilbert <dgilbert@redhat.com> writes:

> * Alex Bennée (alex.bennee@linaro.org) 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
>
> The problem with VGA is that a VGA page can become mapped and unmapped
> under the control of the guest; somewhere in a low address.  This tends
> to break hugepage mappings.
> For vhost-user, and in particular vhost-user-postcopy this means it
> fails the mapping on the vhost-user client.
>
> However the other problem is that with vhost-user, the vhost-user client
> is changing memory; and won't mark the pages as dirty - except for
> migration (I'm not clear if vhost kernel does this).

For virtio this shouldn't be a problem because whatever the vhost-user
client writes to should never be read by the guest until it gets kicked
by the client to signal the virtqueue is done.

I guess migration is a fairly moot point given I haven't seen anything
outside of a test declare VHOST_F_LOG_ALL support.

> So TCG won't notice a page that's been changed by the driver; now in
> most cases it's rare for a device to be writing directly into a page
> you're going to execute out of, but it's not unknown.

Not unknown outside of bugs?

So stage 2 of this exercise is limiting the amount of exposed RAM to the
client to just the virtqueues themselves (which is all vhost-user-rpmb
should need).

> So, as it is, any area that's expecting to get non-migration dirty
> notifications is going to be disappointed by a vhost-user backend.

It's not outside the realms of possibility that we could implement
feedback to the softmmu/migration information from a vhost-user client
but for now I think it's safe to assume we are eliding over the issue.

>
> Dave
>
>> 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;
>> +
>> +    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);
>> +        }
>> +        if (dirty_mask & ~handled_dirty) {
>> +            result = DIRTY;
>> +        } else if (dev->vhost_ops->vhost_backend_mem_section_filter &&
>> +            !dev->vhost_ops->vhost_backend_mem_section_filter(dev, section)) {
>> +            result = FILTERED;
>> +        } else {
>> +            result = OK;
>> +        }
>>      }
>>  
>>      trace_vhost_section(section->mr->name, result);
>> -    return result;
>> +    return result == OK;
>>  }
>>  
>>  static void vhost_begin(MemoryListener *listener)
>> -- 
>> 2.20.1
>> 


-- 
Alex Bennée