[Qemu-devel] Re: [PATCH comment tweaked] msix: allow byte and word reading from mmio

[Bernhard Kohl <bernhard.kohl@nsn.com>]

Am 16.11.2010 14:14, schrieb mst@redhat.com:
> Although explicitly disallowed by the PCI spec, some guests read a
> single byte or word from mmio.  Likely a guest OS bug, but I have an OS
> which reads single bytes and it works fine on real hardware.
>
> Signed-off-by: Bernhard Kohl<bernhard.kohl@nsn.com>
> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> ---
>
> OK so it could like something like the below.

Yes, this looks good for me.

>   However, my question is:
> do we need to put this in or can the guest simply be fixed?
>    

I tried to locate the code where the readw occurs,
but not successful. It only occurs during booting our OS,
and the virtio-net driver seems to be OK. With 4 virtio
NICs we have the following readw accesses, thats all!
3 accesses per NIC and the first NIC appears twice.

MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000008
MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000018
MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000028
MSI-X: msix_mmio_readw dev=0x9772c40 addr=0000000000000008
MSI-X: msix_mmio_readw dev=0x9772c40 addr=0000000000000018
MSI-X: msix_mmio_readw dev=0x9772c40 addr=0000000000000028
MSI-X: msix_mmio_readw dev=0x977dc38 addr=0000000000000008
MSI-X: msix_mmio_readw dev=0x977dc38 addr=0000000000000018
MSI-X: msix_mmio_readw dev=0x977dc38 addr=0000000000000028
MSI-X: msix_mmio_readw dev=0x9788d90 addr=0000000000000008
MSI-X: msix_mmio_readw dev=0x9788d90 addr=0000000000000018
MSI-X: msix_mmio_readw dev=0x9788d90 addr=0000000000000028
MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000008
MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000018
MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000028

Is it possible to add a stack back tace printing to the
readw function?

>   hw/msix.c |   31 +++++++++++++++++++++++++++----
>   1 files changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/hw/msix.c b/hw/msix.c
> index f66d255..38dff59 100644
> --- a/hw/msix.c
> +++ b/hw/msix.c
> @@ -102,10 +102,28 @@ static uint32_t msix_mmio_readl(void *opaque, target_phys_addr_t addr)
>       return pci_get_long(page + offset);
>   }
>
> -static uint32_t msix_mmio_read_unallowed(void *opaque, target_phys_addr_t addr)
> + /* Note:
> +  * PCI spec requires that all MSI-X table accesses are either DWORD or QWORD,
> +  * size aligned.  Some guests seem to violate this rule for read accesses,
> +  * performing single byte reads.  Since it's easy to support this, let's do so.
> +  * Also support 16 bit size aligned reads, just in case.
> +  */
> +static uint32_t msix_mmio_readw(void *opaque, target_phys_addr_t addr)
>   {
> -    fprintf(stderr, "MSI-X: only dword read is allowed!\n");
> -    return 0;
> +    PCIDevice *dev = opaque;
> +    unsigned int offset = addr&  (MSIX_PAGE_SIZE - 1)&  ~0x1;
> +    void *page = dev->msix_table_page;
> +
> +    return pci_get_word(page + offset);
> +}
> +
> +static uint32_t msix_mmio_readb(void *opaque, target_phys_addr_t addr)
> +{
> +    PCIDevice *dev = opaque;
> +    unsigned int offset = addr&  (MSIX_PAGE_SIZE - 1);
> +    void *page = dev->msix_table_page;
> +
> +    return pci_get_byte(page + offset);
>   }
>
>   static uint8_t msix_pending_mask(int vector)
> @@ -192,6 +210,11 @@ static void msix_mmio_writel(void *opaque, target_phys_addr_t addr,
>       msix_handle_mask_update(dev, vector);
>   }
>
> +/* PCI spec:
> + * For all accesses to MSI-X Table and MSI-X PBA fields, software must use
> + * aligned full DWORD or aligned full QWORD transactions; otherwise, the result
> + * is undefined.
> + */
>   static void msix_mmio_write_unallowed(void *opaque, target_phys_addr_t addr,
>                                         uint32_t val)
>   {
> @@ -203,7 +226,7 @@ static CPUWriteMemoryFunc * const msix_mmio_write[] = {
>   };
>
>   static CPUReadMemoryFunc * const msix_mmio_read[] = {
> -    msix_mmio_read_unallowed, msix_mmio_read_unallowed, msix_mmio_readl
> +    msix_mmio_readb, msix_mmio_readw, msix_mmio_readl
>   };
>
>   /* Should be called from device's map method. */
>