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

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Message-ID: <646427f9-7bed-43aa-6aba-9b5e82eecb68@nvidia.com>
Date:   Fri, 28 Sep 2018 14:10:26 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Gerd Hoffmann <kraxel@...hat.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



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?

>> 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.
> 

Ok.

Thanks,
Kirti

> cheers,
>   Gerd
>