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

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

On Tue, Feb 13, 2024 at 11:50:34AM +0000, Srujana Challa wrote:
> > 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;
>     }

Yes but not everyone uses that right? Plain virtio in software with
no tricks doesn't care?


> > 
> > > ---
> > >  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