Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit width when platform is not set

[Julien Grall <julien@xxxxxxx>]

On 20/08/2021 03:04, Wei Chen wrote:
> Hi Julien,
Hi Wei,

> > -----Original Message-----
> > From: Julien Grall <julien@xxxxxxx>
> > Sent: 2021年8月19日 21:28
> > To: Wei Chen <Wei.Chen@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> > sstabellini@xxxxxxxxxx; jbeulich@xxxxxxxx
> > Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
> > Subject: Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit width
> > when platform is not set
> >
> > Hi,
> >
> > On 11/08/2021 11:23, Wei Chen wrote:
> > > From: Hongda Deng <Hongda.Deng@xxxxxxx>
> > >
> > > In current code, arch_get_dma_bitsize will return 32 when platorm
> > > or platform->dma_bitsize is not set. It's not resonable, for Arm,
> > s/resonable/reasonable/
> Ok
>
> > > we don't require to reserve DMA memory. So we set dma_bitsize always
> > > be 0. In NO-NUMA system, arch_get_dma_bitsize will not be invoked,
> > > so dma_bitsize will not be overrided by this function.
> > arch_get_dma_bitsize() is also used to allocate dom0 memory. We need to
> > be able to allocate some DMA-able memory that can be used by every devices.
> >
> > > But in NUMA
> > > system, once the online nodes are greater than 1, this function will
> > > be invoked. The dma_bitsize will be limited to 32. That means, only
> > > first 4GB memory can be used for DMA. But that's against our hardware
> > > design. We don't have that kind of restriction on hardware.
> > What do you mean by "hardware design"? Are you referring to the server
> > you boot Xen on?
> Yes. I will change it to some neutral words. something like:
> "But that could not reflect some hardware's real DMA ability. They may not
> have kind of restriction on hardware." ?
The thing is DMA ability is not about the platform itself. It is more 
about the devices (this could just be a PCI card you just plugged). What 
you seem to suggest is no-one will ever plug such card on your platform. 
Is that correct?
> > Anyway, there are plenty of platform out that have devices which can't
> > DMA into memory above 32-bit. On RPI, this is even lower (30-bit).
> >
> > So I would be cautious to change the default limit.
> How about return 0 when platform doesn't specify the limit?
> In my opinion, arbitrary to give 32 on AArch64 doesn't make sense.
We have to care about the common use-case. We added this restriction a 
few years ago because we had a few reports of users using only 32-bit 
DMA capable device.
> But as you mentioned, if Xen is running on a platform with DMA limitation,
> but we have not listed this platform in Xen supported list, Xen cannot
> get DMA limit from platform->dma_bitsize. In this case, return 0 will
> also cause some issue.
>
> > At the moment, the only place on Arm where we need DMA-able memory is
> > for dom0. This is allocated at boot and can't change afterwards (for now).
> For Dom0, we squash the patch#5 into this patch?
Let me answer with another question. Why should we modify the Arm code 
rather than the common code? IOW...
> > So I would explore to remove the NUMA check for drop the DMA zone. FAOD,
> > both suggestion are for Arm only. For x86, they need to be kept.
> Without introducing new flag, such as lowmem_for_dma, it's a little
> hard to skip the numa node check. Unless we crudely add #ifdef ARCH to
> common code, which is not what we want to see ...
>        if ( !dma_bitsize && (num_online_nodes() > 1) )
>            dma_bitsize = arch_get_dma_bitsize();
... Why do you think we need this check on Arm when NUMA is enabled?

We can discuss how to remove it once this is answered.

Cheers,

--
Julien Grall