RE: [EXT] Re: [PATCH] virtio-pci: correctly set virtio pci queue mem multiplier

[Srujana Challa]

> Subject: [EXT] Re: [PATCH] virtio-pci: correctly set virtio pci queue mem
> multiplier
> 
> External Email
> 
> ----------------------------------------------------------------------
> On Mon, Feb 12, 2024 at 01:22:33PM +0530, Srujana Challa wrote:
> > Currently, virtio_pci_queue_mem_mult function returns 4K when
> > VIRTIO_PCI_FLAG_PAGE_PER_VQ is set. But this is not correct when host
> > has page size as 64K.
> > This patch fixes the same.
> >
> > Signed-off-by: Srujana Challa <schalla@marvell.com>
> 
> You can't tweak guest visible values like this without compat machinery. It's
> also going to consume a ton more phys memory - can this break any configs?
> Why is this a problem? Just with vdpa?

We are observing the issue with vdpa when host has page size of 64K. We haven't
verified any other backends. I think, any backend that uses below API would fail
if host has page size other than 4K right?
And also as per VIRTIO_PCI_FLAG_PAGE_PER_VQ, it should be equal to
page_size right?

static int virtio_pci_set_host_notifier_mr(DeviceState *d, int n,
                                           MemoryRegion *mr, bool assign)
{
    VirtIOPCIProxy *proxy = to_virtio_pci_proxy(d);
    int offset;

    if (n >= VIRTIO_QUEUE_MAX || !virtio_pci_modern(proxy) ||
        virtio_pci_queue_mem_mult(proxy) != memory_region_size(mr)) {
        return -1;
    }

> 
> > ---
> >  hw/virtio/virtio-pci.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c index
> > e433879542..028df99991 100644
> > --- a/hw/virtio/virtio-pci.c
> > +++ b/hw/virtio/virtio-pci.c
> > @@ -316,12 +316,10 @@ static bool
> virtio_pci_ioeventfd_enabled(DeviceState *d)
> >      return (proxy->flags & VIRTIO_PCI_FLAG_USE_IOEVENTFD) != 0;  }
> >
> > -#define QEMU_VIRTIO_PCI_QUEUE_MEM_MULT 0x1000
> > -
> >  static inline int virtio_pci_queue_mem_mult(struct VirtIOPCIProxy
> > *proxy)  {
> >      return (proxy->flags & VIRTIO_PCI_FLAG_PAGE_PER_VQ) ?
> > -        QEMU_VIRTIO_PCI_QUEUE_MEM_MULT : 4;
> > +        qemu_real_host_page_size()  : 4;
> >  }
> >
> >  static int virtio_pci_ioeventfd_assign(DeviceState *d, EventNotifier
> > *notifier,
> > --
> > 2.25.1