Thank you, Tim and Stefano.
On Mon, Jun 27, 2011 at 7:23 PM, Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> wrote:
> On Mon, 27 Jun 2011, Tim Deegan wrote:
>> At 17:26 +0800 on 27 Jun (1309195596), Wei Liu wrote:
>> > > You need to arrange for the Xen specific bit of the virtio frontend to
>> > > do the p2m translation. All externally visible addresses from a PV guest
>> > > must be in MFN space.
>> > >
>> >
>> > That's not a good approach. Dirvers reside on a higher level from the
>> > "Xen specific bit". Doing this kind of translation will break their
>> > generality. But there seems no other way to get this job done...
>>
>> This is a general problem with operating systems: bus addresses are not
>> always the same as CPU memory addresses (even though in the easy case of
>> a simple x86 PC they often are). This is why OSes already have
>> interfaces to allocate DMA-able memory and to translate between the
>> address that CPU will map and the one the peripheral should use. If
>> your driver's not using the right address-space to program its hardware
>> it is broken; the Xen PV model is just an extreme case of this.
>
> Indeed.
>
> When passing addresses to devices, drivers should use the DMA APIs to
> make sure that proper address conversions take place (see
> Documentation/DMA-API-HOWTO.txt).
> On Xen the swiotlb implementation takes care of doing pfn to mfn
> conversions as well.
> For example, to allocate a consistent buffer that is addressable by both
> a device and the cpu, the correct API is dma_alloc_coherent, that
> returns two values: the virtual address which you can use to access it
> from the CPU and dma_handle which you pass to the device.
>
This is the first time to notice swiotlb TBH. It seems to be a good
start for me.
After skimming the code in $KERNEL/drivers/xen/swiotlb-xen.c, I have
further questions.
It seems that swiotlb is used for PCI passthrough in PV, I'm wondering
if paravirt drivers ever use it.
> I can very well imagine that virtio drivers are not doing any of this
> because they didn't need to, but it would be probably OK to submit a
> patch to convert the drivers to use the proper APIs.
>
No, they are not doing this.
Do you mean that I replace every allocation/deallocation of virtio
drivers scatter buffer with DMA API?
Then, with 'swiotlb=force' in pv guest kernel command line, these
DMA-API will call xen-swiotlb and get proper addresses?
If I don't add 'swiotlb=force', which underlying implementation will
be used? And, which DMA-API will be used in hvm case? will this change
affect hvm guest?
Wei.
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