Hi Julien,
> -----Original Message-----
> From: Julien Grall <[email protected]>
> Sent: 2021年8月19日 21:28
> To: Wei Chen <[email protected]>; [email protected];
> [email protected]; [email protected]
> Cc: Bertrand Marquis <[email protected]>
> 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 <[email protected]>
> >
> > 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." ?
> 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.
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?
> 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();
> > Only
> > platform setting can override dma_bitsize. So in this patch, we
> > return default dma_bitsize, when platform and platorm->dma_bitsize
> > are not set.
> >
> > Signed-off-by: Wei Chen <[email protected]>
> > Signed-off-by: Hongda Deng <[email protected]>
> > ---
> > xen/arch/arm/platform.c | 4 +++-
> > xen/common/page_alloc.c | 2 +-
> > 2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/xen/arch/arm/platform.c b/xen/arch/arm/platform.c
> > index 4db5bbb4c5..0a27fef9a4 100644
> > --- a/xen/arch/arm/platform.c
> > +++ b/xen/arch/arm/platform.c
> > @@ -27,6 +27,7 @@ extern const struct platform_desc _splatform[],
> _eplatform[];
> > /* Pointer to the current platform description */
> > static const struct platform_desc *platform;
> >
> > +extern unsigned int dma_bitsize;
> >
> > static bool __init platform_is_compatible(const struct platform_desc
> *plat)
> > {
> > @@ -157,7 +158,8 @@ bool platform_device_is_blacklisted(const struct
> dt_device_node *node)
> >
> > unsigned int arch_get_dma_bitsize(void)
> > {
> > - return ( platform && platform->dma_bitsize ) ? platform-
> >dma_bitsize : 32;
> > + return ( platform && platform->dma_bitsize ) ? platform-
> >dma_bitsize
> > + : dma_bitsize;
> > }
> >
> > /*
> > diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
> > index 958ba0cd92..0f0cae5a4e 100644
> > --- a/xen/common/page_alloc.c
> > +++ b/xen/common/page_alloc.c
> > @@ -227,7 +227,7 @@ static bool __read_mostly scrub_debug;
> > * Bit width of the DMA heap -- used to override NUMA-node-first.
> > * allocation strategy, which can otherwise exhaust low memory.
> > */
> > -static unsigned int dma_bitsize;
> > +unsigned int dma_bitsize;
> > integer_param("dma_bits", dma_bitsize);
> >
> > /* Offlined page list, protected by heap_lock. */
> >
>
> Cheers,
>
> --
> Julien Grall
