Re: [PATCH RFC v2 09/16] vfio-user: region read/write

[John Levon]

On Fri, Sep 10, 2021 at 06:07:56AM +0000, John Johnson wrote:

> >>> On Mon, Aug 16, 2021 at 09:42:42AM -0700, Elena Ufimtseva wrote:
> >>> 
> >>>> +int vfio_user_region_write(VFIODevice *vbasedev, uint32_t index,
> >>>> +                           uint64_t offset, uint32_t count, void *data)
> >>>> +{
> >>>> +    g_autofree VFIOUserRegionRW *msgp = NULL;
> >>>> +    int size = sizeof(*msgp) + count;
> >>>> +
> >>>> +    msgp = g_malloc0(size);
> >>>> +    vfio_user_request_msg(&msgp->hdr, VFIO_USER_REGION_WRITE, size,
> >>>> +                          VFIO_USER_NO_REPLY);
> >>> 
> >>> Mirroring 
> >>> https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_oracle_qemu_issues_10&d=DwIGaQ&c=s883GpUCOChKOHiocYtGcg&r=v7SNLJqx7b9Vfc7ZO82Wg4nnZ8O5XkACFQ30bVKxotI&m=PJ390CfKPdTFUffSi02whMSqey2en8OJv7dm9VAQKI0&s=Mfp1xRKET3LEucEeZwUVUvSJ3V0zzGEktOzFwMsTfEE&e=
> >>>   here for visibility:
> >>> 
> >>> Currently, vfio_user_region_write uses VFIO_USER_NO_REPLY unconditionally,
> >>> meaning essentially all writes are posted. But that shouldn't be the 
> >>> case, for
> >>> example for PCI config space, where it's expected that writes will wait 
> >>> for an
> >>> ack before the VCPU continues.
> >> 
> >>    I’m not sure following the PCI spec (mem writes posted, config & IO
> >> are not) completely solves the issue if the device uses sparse mmap.  A 
> >> store
> >> to went over the socket can be passed by a load that goes directly to 
> >> memory,
> >> which could break a driver that assumes a load completion means older 
> >> stores
> >> to the same device have also completed.
> > 
> > Sure, but sparse mmaps are under the device's control - so wouldn't that be
> > something of a "don't do that" scenario?
> 
>       The sparse mmaps are under the emulation program’s control, but it
> doesn’t know what registers the guest device driver is using to force stores
> to complete.  The Linux device drivers doc on kernel.org just says the driver
> must read from the same device.

Sure, but any device where that is important wouldn't use the sparse mmaps, no?

There's no other alternative.

regards
john