Re: [RFC PATCH] tests/qtest: properly initialise the vring used idx

[Peter Maydell]

On Wed, 6 Apr 2022 at 18:36, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> Eric noticed while attempting to enable the vhost-user-blk-test for
> Aarch64 that that things didn't work unless he put in a dummy
> guest_malloc() at the start of the test. Without it
> qvirtio_wait_used_elem() would assert when it reads a junk value for
> idx resulting in:
>
>   qvirtqueue_get_buf: idx:2401 last_idx:0
>   qvirtqueue_get_buf: 0x7ffcb6d3fe74, (nil)
>   qvirtio_wait_used_elem: 3000000/0
>   ERROR:../../tests/qtest/libqos/virtio.c:226:qvirtio_wait_used_elem: 
> assertion failed (got_desc_idx == desc_idx): (50331648 == 0)
>   Bail out! 
> ERROR:../../tests/qtest/libqos/virtio.c:226:qvirtio_wait_used_elem: assertion 
> failed (got_desc_idx == desc_idx): (50331648 == 0)
>
> What was actually happening is the guest_malloc() effectively pushed
> the allocation of the vring into the next page which just happened to
> have clear memory. After much tedious tracing of the code I could see
> that qvring_init() does attempt initialise a bunch of the vring
> structures but skips the vring->used.idx value. It is probably not
> wise to assume guest memory is zeroed anyway. Once the ring is
> properly initialised the hack is no longer needed to get things
> working.

Guest memory is generally zero at startup. Do we manage to
hit the bit of memory at the start of the virt machine's RAM
where we store the DTB ? (As you say, initializing the data
structures is the right thing anyway.)

thanks
-- PMM