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

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Message-ID: <20180928064941.4fac51ef@t450s.home>
Date:   Fri, 28 Sep 2018 06:49:41 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>,
        <intel-gvt-dev@...ts.freedesktop.org>, <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

On Fri, 28 Sep 2018 14:10:26 +0530
Kirti Wankhede <kwankhede@...dia.com> wrote:

> On 9/28/2018 11:10 AM, Gerd Hoffmann wrote:
> >>> +	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
> >   
> 
> Ok.
> Can you add a comment in vfio.h that this region is non-mmappable?

No, region access mechanisms are self describing through the region
info ioctl, it's left to the implementation to decide what to support.
The region definition should not impose such a restriction.  Thanks,

Alex