Re: Questions about the VFIO BAR region

[Li Qiang <liq3ea@gmail.com>]

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Alex Williamson <alex.williamson@redhat.com> 于2019年11月5日周二 上午2:49写道：

> On Tue, 5 Nov 2019 00:40:39 +0800
> Li Qiang <liq3ea@163.com> wrote:
>
> > Hello Alex, Auger and all,
> >
> > I have a question about the VFIO virtual device BAR.
> >
> > In vfio_region_setup, it initialize a ‘region->mem’ MR and set its ops
> to ‘vfio_regions_ops’.
> > In ‘vfio_region_mmap’, it maps the physical device’s MMIO to QEMU’s
> virtual address space
> > as a raw MR ‘region->mmaps[i].mem’.
> > And also it set the latter MR as a subregion of the first one.
> >
> > So when the guest accesses the BAR, it will direct go to the physical
> device’s BAR.
> > My question is here:
> > When the qemu will use the ‘vfio_regions_ops’ to read/write the BAR?
> > Also whey in the last of ‘vfio_region_write/read’ we need to call
> ‘vbasedev->ops->vfio_eoi(vbasedev);’?
>
> We support:
>
>  a) sparse mmaps where the entire BAR is not covered by an mmap
>

Got.



>  b) quirks, which layer on top of the mmaps to provide virtualized
>     access
>

Do you mean like in 'vfio_probe_ati_bar4_quirk', register a high priority
subregion of VFIORegion.mem.
So when the guest write the BAR, vfio_regions_ops will be used. Here
'quirks' do you mean such things?

static void vfio_probe_ati_bar4_quirk(VFIOPCIDevice *vdev, int nr)
{
    VFIOQuirk *quirk;
    VFIOConfigWindowQuirk *window;

    ...
    memory_region_init_io(window->addr_mem, OBJECT(vdev),
                          &vfio_generic_window_address_quirk, window,
                          "vfio-ati-bar4-window-address-quirk", 4);
    memory_region_add_subregion_overlap(vdev->bars[nr].region.mem,
                                        window->address_offset,
                                        window->addr_mem, 1);
   ...
}



>  c) INTx emulation which disables mmaps MRs in order to detect device
>     access as a generic mechanism for inferring interrupt
>     acknowledgment.
>

In the above two cases, in 'vfio_region_write/read' we always access the
physical device's BAR.
So as far as I can understand, the physical device(sometimes) will trigger
interrupts. And the responsible of clear it
will be by the 'guest'. So I can't understand why there calls
'vbasedev->ops->vfio_eoi'. Could you please give me an
example.


Thanks,
Li Qiang



>
> The latter being the reason we call vfio_eoi.  Thanks,
>
> Alex
>
>

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