Re: [PATCH v7 6/8] tests/qtest: Add GET_SHMEM validation test

[Alyssa Ross <hi@alyssa.is>]

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Albert Esteve <aesteve@redhat.com> writes:

> On Tue, Aug 19, 2025 at 12:42 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>
>> On Mon, Aug 18, 2025 at 12:03:51PM +0200, Albert Esteve wrote:
>> > Improve vhost-user-test to properly validate
>> > VHOST_USER_GET_SHMEM_CONFIG message handling by
>> > directly simulating the message exchange.
>> >
>> > The test manually triggers the
>> > VHOST_USER_GET_SHMEM_CONFIG message by calling
>> > chr_read() with a crafted VhostUserMsg, allowing direct
>> > validation of the shmem configuration response handler.
>>
>> It looks like this test case invokes its own chr_read() function without
>> going through QEMU, so I don't understand what this is testing?
>
> I spent some time trying to test it, but in the end I could not
> instatiate vhost-user-device because it is non user_creatable. I did
> not find any test for vhost-user-device anywhere else either. But I
> had already added most of the infrastructure here so I fallback to
> chr_read() communication to avoid having to delete everything. My
> though was that once we have other devices that use shared memory,
> they could tweak the test to instantiate the proper device and test
> this and the map/unmap operations.
>
> Although after writing this, I think other devices will actually a
> specific layout for their shared memory. So
> VHOST_USER_GET_SHMEM_CONFIG is only ever going to be used by
> vhost-user-device.

FWIW: I'm not so sure — my non-upstream Cloud Hypervisor frontend for
the crosvm vhost-user GPU device[1] uses the equivalent of
VHOST_USER_GET_SHMEM_CONFIG to allow the backend to choose the size of
the shared memory region, and I could imagine that being something other
devices might want to do too?

[1]: https://spectrum-os.org/software/cloud-hypervisor/

> In general, trying to test this patch series has been a headache other
> than trying with external device code I have. If you have an idea that
> I could try to test this, I can try. Otherwise, probably is best to
> remove this commit from the series and wait for another vhost-user
> device that uses map/unmap to land to be able to test it.

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