linux-kernel - Re: [PATCH v3 2/2] vfio: add edid support to mbochs sample driver

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Message-ID: <20180928054056.vjqwxvaj7sjmsozh@sirius.home.kraxel.org>
Date:   Fri, 28 Sep 2018 07:40:56 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.com>
Cc:     intel-gvt-dev@...ts.freedesktop.org,
        Alex Williamson <alex.williamson@...hat.com>,
        kvm@...r.kernel.org, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] vfio: add edid support to mbochs sample driver

> > +	case MBOCHS_EDID_REGION_INDEX:
> > +		ext->base.argsz = sizeof(*ext);
> > +		ext->base.offset = MBOCHS_EDID_OFFSET;
> > +		ext->base.size = MBOCHS_EDID_SIZE;
> > +		ext->base.flags = (VFIO_REGION_INFO_FLAG_READ  |
> > +				   VFIO_REGION_INFO_FLAG_WRITE |
> > +				   VFIO_REGION_INFO_FLAG_CAPS);
> 
> Any reason to not to use _MMAP flag?

There is no page backing this.  Also it is not performance-critical,
edid updates should be rare, so the extra code for mmap support doesn't
look like it is worth it.

Also for the virtual registers (especially link_state) it is probably
useful to have the write callback of the mdev driver called to get
notified about the change.

> How would QEMU side code read this region? will it be always trapped?

qemu uses read & write syscalls (well, pread & pwrite actually).

> If vendor driver sets _MMAP flag, will QEMU side handle that case as well?

The current test branch doesn't, it expects read+write to work.
  https://git.kraxel.org/cgit/qemu/log/?h=sirius/edid-vfio

> I think since its blob, edid could be read by QEMU using one memcpy
> rather than adding multiple memcpy of 4 or 8 bytes.

>From qemu it's a single pwrite syscall actually.  mbochs_write() splits
it into 4 byte writes and calls mbochs_access() for each of them.  One
could probably add a special case for the EDID blob to mbochs_write().
But again: doesn't seem worth the effort given that edid updates should
be a rare event.

cheers,
  Gerd