> -----Original Message-----
> From: Eugenio Perez Martin <[email protected]>
> Sent: 2025年12月19日 16:12
> To: Wafer <[email protected]>
> Cc: [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; Angus Chen
> <[email protected]>
> Subject: Re: [PATCH v3 1/4] vhost: add data structure of virtio indirect
> descriptors in SVQ
>
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>
> On Tue, Dec 16, 2025 at 2:55 AM Wafer Xie <[email protected]> wrote:
> >
> > From: wafer Xie <[email protected]>
> >
> > Using multiple statically allocated buffers to store virtio indirect
> > descriptors
> >
> > Signed-off-by: wafer Xie <[email protected]>
> > ---
> > hw/virtio/vhost-shadow-virtqueue.h | 46
> > ++++++++++++++++++++++++++++++
> > 1 file changed, 46 insertions(+)
> >
> > diff --git a/hw/virtio/vhost-shadow-virtqueue.h
> > b/hw/virtio/vhost-shadow-virtqueue.h
> > index 9c273739d6..b0f236f049 100644
> > --- a/hw/virtio/vhost-shadow-virtqueue.h
> > +++ b/hw/virtio/vhost-shadow-virtqueue.h
> > @@ -23,6 +23,13 @@ typedef struct SVQDescState {
> > * guest's
> > */
> > unsigned int ndescs;
> > +
> > + /*
> > + * Index into the indirect descriptor
> > + * buffer (0 to SVQ_NUM_INDIRECT_BUFS - 1) if using indirect.
> > + * -1 if not using indirect descriptors.
> > + */
> > + int indirect_buf_idx;
> > } SVQDescState;
> >
> > typedef struct VhostShadowVirtqueue VhostShadowVirtqueue; @@ -46,6
> > +53,32 @@ typedef struct VhostShadowVirtqueueOps {
> > VirtQueueAvailCallback avail_handler; } VhostShadowVirtqueueOps;
> >
> > +/**
> > + * State of an indirect descriptor buffer */ typedef enum
> > +SVQIndirectBufState {
> > + SVQ_INDIRECT_BUF_FREED, /* Buffer is free and can be used */
> > + SVQ_INDIRECT_BUF_FREEING, /* Buffer is being freed */ }
> > +SVQIndirectBufState;
> > +
> > +/**
> > + * Pre-allocated indirect descriptor buffer
> > + * Each SVQ has two of these buffers for double-buffering
>
> What does the double buffering improve here compared to just doubling the
> size?
>
Compared to simply doubling the size, double buffering mainly makes indirect
descriptor management easier.
For indirect descriptors, the virtio specification requires the descriptor
table to be contiguous in memory.
The last descriptor in the array cannot be chained with the descriptor at index
0 to form an indirect descriptor table.
>
> > + */
> > +typedef struct SVQIndirectDescBuf {
> > + vring_desc_t *desc; /* Descriptor table size = num_descs) */
> > + hwaddr iova; /* IOVA of the descriptor table */
> > + size_t size; /* Size of the mmap'd region */
> > + SVQIndirectBufState state; /* Current state of this buffer */
> > + uint16_t num_descs; /* Total number of descs (N * vring.num) */
> > + uint16_t freed_descs; /* Number of descriptors available for use
> > */
> > + uint16_t freeing_descs; /* Number of descs being freed from used
> ring */
> > + uint16_t freed_head; /* Next free descriptor index in this
> > buffer */
> > +} SVQIndirectDescBuf;
> > +
> > +/* Number of indirect descriptor buffers per SVQ for double-buffering
> > +*/ #define SVQ_NUM_INDIRECT_BUFS 2
> > +
> > /* Shadow virtqueue to relay notifications */ typedef struct
> > VhostShadowVirtqueue {
> > /* Shadow vring */
> > @@ -96,6 +129,19 @@ typedef struct VhostShadowVirtqueue {
> > /* Caller callbacks opaque */
> > void *ops_opaque;
> >
> > + /*
> > + * Pre-allocated indirect descriptor buffers for double-buffering.
> > + * These are allocated when SVQ starts if
> VIRTIO_RING_F_INDIRECT_DESC
> > + * is negotiated.
> > + */
> > + SVQIndirectDescBuf indirect_bufs[SVQ_NUM_INDIRECT_BUFS];
> > +
> > + /* Index of the current active indirect buffer, -1 if not using */
> > + int current_indirect_buf;
> > +
> > + /* Whether indirect descriptors are enabled */
> > + bool indirect_enabled;
> > +
> > /* Next head to expose to the device */
> > uint16_t shadow_avail_idx;
> >
> > --
> > 2.48.1
> >
