On 19/09/2016 19:15, Michael S. Tsirkin wrote:
> On Mon, Sep 19, 2016 at 02:01:57PM +0200, Paolo Bonzini wrote:
>> The main issue with VRING_DESC_F_INDIRECT, at least as it is implemented
>> in Linux, is the cost of allocating and freeing the indirect descriptor.
>> But if you know that the number of descriptor is fixed (e.g. 2 for
>> request header + data) you can pre-allocate the descriptors with a 1:1
>> relationship with the entries of the ring buffer. This might be useful
>> for the virtio-net tx ring.
>
> Yes, I've been thinking the same thing. Is there any advantage
> to have the indirect addresses pre-programmed as
> opposed to stored in the descriptors?
>
> In theory
> if (desc.flags & indirect)
> use(*address[i])
>
> might be speculated as compared to
> if (desc.flags & indirect)
> use(*desc.addr)
> which can't.
> But OTOH this might add to cache pressure as desc is already in cache
> anyway.
Yes, I think this would be the worst of both worlds. :)
If the driver knows it has 2 descriptors per request, it could allocate
a big block in advance and do for example
struct {
// pre-allocated and pre-initialized
// to point to ->hdr
struct virtio_desc req_desc;
// only flags prefilled
struct virtio_desc data_desc;
struct req_header hdr;
} *indirect_data;
i = index++ & (size - 1);
indirect_addr = &indirect_data[i];
indirect_data[i]->data_desc.addr = this_data_addr;
indirect_data[i]->data_desc.len = this_data_len;
// ... fill in indirect_data[i]->hdr ...
desc[i].addr = indirect_addr;
smp_wmb();
desc[i].index = i | DESC_HW;
where there is no need for an array of descriptor addresses,
&indirect_data[i]. As long as the indirect_data size is a power of 2,
indirect_data needn't even be physically contiguous.
But I prefer all these tricks to be hidden within the driver. It seems
a good idea in the beginning to rig the device to second-guess what a
driver could do, but then it makes things awkward. (This is also why
I'd rather get rid of VRING_DESC_F_NEXT).
> If all descriptors are known ahead of the time to be indirect we could
> pack more of them per cache line by dropping addr (and maybe len?).
> This might work well for virtio blk.
Nope, virtio-blk doesn't know at all the size of the sg list (for
small-I/O workloads it is always 2, but potentially unlimited). For the
above ideas I was thinking of the virtio tx header (but only for
MTU=1500, as Stephen pointed out).
Paolo
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