Re: [PATCH v2] hw/virtio/vhost: re-factor vhost-section and allow DIRTY_MEMORY_CODE

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 04, 2020 at 02:40:22PM +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.
> 
> 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
> 
> We expand the comment to explain that kernel based vhost has specific
> support for migration tracking.
> 
> 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>
> 
> ---
> v2
>   - drop enum, add trace_vhost_reject_section
>   - return false at any fail point
>   - unconditionally add DIRTY_MEMORY_CODE to handled cases
>   - slightly re-word the explanatory comment and commit message
> ---
>  hw/virtio/vhost.c      | 55 ++++++++++++++++++++++++++++++------------
>  hw/virtio/trace-events |  3 ++-
>  2 files changed, 41 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index aff98a0ede5..120c0cc747b 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 +404,48 @@ 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;
> +    MemoryRegion *mr = section->mr;
> +
> +    if (memory_region_is_ram(mr) && !memory_region_is_rom(mr)) {
> +        uint8_t dirty_mask = memory_region_get_dirty_log_mask(mr);
> +        uint8_t handled_dirty;
> +
> +        /*
> +         * Kernel based vhost doesn't handle any block which is doing
> +         * dirty-tracking other than migration for which it has
> +         * specific logging support. However for TCG the kernel never
> +         * gets involved anyway so we can also ignore it's
> +         * self-modiying code detection flags.
> +         */
> +        handled_dirty = (1 << DIRTY_MEMORY_MIGRATION);
> +        handled_dirty |= (1 << DIRTY_MEMORY_CODE);

I'd just rewrite it in a single statement:

         handled_dirty = (1 << DIRTY_MEMORY_MIGRATION) |
                         (1 << DIRTY_MEMORY_CODE);


>  
> -    if (result && dev->vhost_ops->vhost_backend_mem_section_filter) {
> -        result &=
> -            dev->vhost_ops->vhost_backend_mem_section_filter(dev, section);
> -    }
> +        if (dirty_mask & ~handled_dirty) {
> +            trace_vhost_reject_section(mr->name, 1);
> +            return false;
> +        }
> +
> +        if (dev->vhost_ops->vhost_backend_mem_section_filter &&
> +            !dev->vhost_ops->vhost_backend_mem_section_filter(dev, section)) {
> +            trace_vhost_reject_section(mr->name, 2);
> +            return false;
> +        }
>  
> -    trace_vhost_section(section->mr->name, result);
> -    return result;
> +        trace_vhost_section(mr->name);
> +        return true;
> +    } else {
> +        trace_vhost_reject_section(mr->name, 3);
> +        return false;
> +    }
>  }
>  
>  static void vhost_begin(MemoryListener *listener)
> diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
> index 84ecb85d445..22427126b97 100644
> --- a/hw/virtio/trace-events
> +++ b/hw/virtio/trace-events
> @@ -5,7 +5,8 @@ vhost_commit(bool started, bool changed) "Started: %d Changed: %d"
>  vhost_region_add_section(const char *name, uint64_t gpa, uint64_t size, uint64_t host) "%s: 0x%"PRIx64"+0x%"PRIx64" @ 0x%"PRIx64
>  vhost_region_add_section_merge(const char *name, uint64_t new_size, uint64_t gpa, uint64_t owr) "%s: size: 0x%"PRIx64 " gpa: 0x%"PRIx64 " owr: 0x%"PRIx64
>  vhost_region_add_section_aligned(const char *name, uint64_t gpa, uint64_t size, uint64_t host) "%s: 0x%"PRIx64"+0x%"PRIx64" @ 0x%"PRIx64
> -vhost_section(const char *name, int r) "%s:%d"
> +vhost_section(const char *name) "%s"
> +vhost_reject_section(const char *name, int d) "%s:%d"
>  vhost_iotlb_miss(void *dev, int step) "%p step %d"

Looks good otherwise, thanks!


>  # vhost-user.c
> -- 
> 2.20.1