Re: [Qemu-devel] [RFC v2 6/6] hw/arm: Populate non-contiguous memory regions

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Peter Maydell [mailto:address@hidden
> Sent: 15 June 2018 17:33
> To: Auger Eric <address@hidden>
> Cc: Andrew Jones <address@hidden>; Shameerali Kolothum Thodi
> <address@hidden>; address@hidden; qemu-
> address@hidden; Jonathan Cameron <address@hidden>;
> Linuxarm <address@hidden>; address@hidden;
> Zhaoshenglong <address@hidden>; address@hidden
> Subject: Re: [Qemu-devel] [RFC v2 6/6] hw/arm: Populate non-contiguous
> memory regions
> 
> On 15 June 2018 at 17:13, Auger Eric <address@hidden> wrote:
> > On 06/15/2018 05:54 PM, Peter Maydell wrote:
> >> Why should the VM ever care about where the address regions in the
> >> host happen to be when it comes to where it's putting its RAM
> >> in the VM's address layout? I feel like I'm missing something here...
> 
> > The VM cannot use RAM GPA that matches assigned device reserved IOVA
> > regions. When you assign a device, the whole VM RAM is mapped in the
> > physical IOMMU and IOVA corresponds to GPA. but sometimes some IOVA
> > cannot be used due to the host specificities and especially iommu
> > peculiarities. Some IOVAs may be simply "bypassed" by the iommu, like on
> > x86 and also with some ARM SMMU integration. In such a case the DMA
> > accesses have no chance to reach the guest RAM as expected. Hope it
> > clarifies.
> 
> Hmm. We don't want to have the address layout of
> the VM have to cope with all the various oddities that host
> systems might have. Is this kind of thing common, or rare?
> I'd much rather just say "we don't support device passthrough
> on that sort of system".
>

As far as I know on ARM64 platforms this is not very rare mainly because of
the phys MSI doorbell and the fact that memory map is not standardized. This
was discussed in LPC sometime back[1] and a sysfs interface was provided to
report these reserved regions. And later Alex commented that vfio needs a more
robust way of identifying and reporting these regions and that’s how the vfio 
kernel
patch series[2] was introduced on which this qemu series is based on.

Thanks,
Shameer

[1] https://lkml.org/lkml/2016/11/7/935

[2] https://lkml.org/lkml/2018/4/18/293