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Subject: Re: [PATCH 0/2] virtio: decouple protected guest RAM form
 VIRTIO_F_IOMMU_PLATFORM
To:     Halil Pasic <pasic@linux.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@lst.de>, linux-s390@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Viktor Mihajlovski <mihajlov@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Ram Pai <linuxram@us.ibm.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        David Gibson <david@gibson.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        Michael Mueller <mimu@linux.ibm.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <1b2673e7-56ff-7d69-af2d-503a97408d95@redhat.com>
Date:   Tue, 25 Feb 2020 11:30:16 +0800
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On 2020/2/24 =E4=B8=8B=E5=8D=889:56, Halil Pasic wrote:
> On Mon, 24 Feb 2020 17:26:20 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> That's better.
>>
>> How about attached?
>>
>> Thanks
> Thanks Jason! It does avoid the translation overhead in vhost.
>
> Tested-by: Halil Pasic <pasic@linux.ibm.com>
>
> Regarding the code, you fence it in virtio-net.c, but AFAIU this featur=
e
> has relevance for other vhost devices as well. E.g. what about vhost
> user? Would it be the responsibility of each virtio device to fence thi=
s
> on its own?


Yes, it looks to me it's better to do that in virtio_set_features_nocheck=
()


>
> I'm also a bit confused about the semantics of the vhost feature bit
> F_ACCESS_PLATFORM. What we have specified on virtio level is:
> """
> This feature indicates that the device can be used on a platform where
> device access to data in memory is limited and/or translated. E.g. this
> is the case if the device can be located behind an IOMMU that translate=
s
> bus addresses from the device into physical addresses in memory, if the
> device can be limited to only access certain memory addresses or if
> special commands such as a cache flush can be needed to synchronise dat=
a
> in memory with the device. Whether accesses are actually limited or
> translated is described by platform-specific means. If this feature bit
> is set to 0, then the device has same access to memory addresses
> supplied to it as the driver has. In particular, the device will always
> use physical addresses matching addresses used by the driver (typically
> meaning physical addresses used by the CPU) and not translated further,
> and can access any address supplied to it by the driver. When clear,
> this overrides any platform-specific description of whether device
> access is limited or translated in any way, e.g. whether an IOMMU may b=
e
> present.
> """
>
> I read this like the addresses may be IOVAs which require
> IMMU translation or GPAs which don't.
>
> On the vhost level however, it seems that F_IOMMU_PLATFORM means that
> vhost has to do the translation (via IOTLB API).


Yes.


>
> Do I understand this correctly? If yes, I believe we should document
> this properly.


Good point. I think it was probably wrong to tie F_IOMMU_PLATFORM to=20
IOTLB API. Technically IOTLB can work with GPA->HVA mapping. I=20
originally use a dedicated feature bit (you can see that from commit=20
log), but for some reason Michael tweak it to virtio feature bit. I=20
guess it was probably because at that time there's no way to specify e.g=20
backend capability but now we have VHOST_GET_BACKEND_FEATURES.

For now, it was probably too late to fix that but document or we can add=20
the support of enabling IOTLB via new backend features.


>
> BTW I'm still not 100% on the purpose and semantics of the
> F_ACCESS_PLATFORM feature bit. But that is a different problem.


Yes, I aggree that we should decouple the features that does not belongs=20
to device (protected, encrypted, swiotlb etc) from F_IOMMU_PLATFORM. But=20
Michael and other have their points as well.

Thanks


>
> Regards,
> Halil
>