Re: [PATCH] x86/xen/mmu: Increase MAX_CONTIG_ORDER

[Jürgen Groß <jgross@xxxxxxxx>]

On 18.12.24 12:11, Thierry Escande wrote:
> On 12/12/2024 12:09, Juergen Gross wrote:
> > On 12.12.24 11:22, Jan Beulich wrote:
> > > On 11.12.2024 19:20, Thierry Escande wrote:
> > > > Hi Jan,
> > > >
> > > > On 09/12/2024 11:04, Jan Beulich wrote:
> > > > > On 04.12.2024 18:14, Thierry Escande wrote:
> > > > > > With change 9f40ec84a797 (xen/swiotlb: add alignment check for dma
> > > > > > buffers), the driver mpt3sas fails to load because it cannot allocate
> > > > > > its DMA pool for an allocation size of ~2,3 MBytes. This is because
> > > > > > the
> > > > > > alignement check added by 9f40ec84a797 fails and
> > > > > > xen_swiotlb_alloc_coherent() ends up calling
> > > > > > xen_create_contiguous_region() with a size order of 10 which is too
> > > > > > high
> > > > > > for the current max value.
> > > > > >
> > > > > > This patch increases the MAX_CONTIG_ORDER from 9 to 10 (4MB) to allow
> > > > > > such allocations.
> > > > > >
> > > > > > Signed-off-by: Thierry Escande <thierry.escande@xxxxxxxxxx>
> > > > > > ---
> > > > > >    arch/x86/xen/mmu_pv.c | 2 +-
> > > > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> > > > > > index 55a4996d0c04..7f110740e1a2 100644
> > > > > > --- a/arch/x86/xen/mmu_pv.c
> > > > > > +++ b/arch/x86/xen/mmu_pv.c
> > > > > > @@ -2200,7 +2200,7 @@ void __init xen_init_mmu_ops(void)
> > > > > >    }
> > > > > >      /* Protected by xen_reservation_lock. */
> > > > > > -#define MAX_CONTIG_ORDER 9 /* 2MB */
> > > > > > +#define MAX_CONTIG_ORDER 10 /* 4MB */
> > > > > >    static unsigned long discontig_frames[1<<MAX_CONTIG_ORDER];
> > > > > While lacking respective commentary, bumping this value imo also
> > > > > needs to
> > > > > take into account Xen itself, at least commit-message-wise. The
> > > > > bumping is
> > > > > fine for Dom0 in any event. It is also fine for DomU-s with the
> > > > > defaults
> > > > > built into the hypervisor (orders 12 and 10 respectively for x86 and
> > > > > Arm),
> > > > > yet especially for Arm (and in the future PPC and RISC-V) any further
> > > > > bumping would be less straightforward.
> > > > Thanks for pointing this out. On the Xen side, CONFIG_CTLDOM_MAX_ORDER
> > > > and CONFIG_HWDOM_MAX_ORDER seem big enough on all architectures. But I
> > > > see CONFIG_DOMU_MAX_ORDER set to 9 (also all archs). Won't that be a
> > > > problem for drivers trying to allocate more than that from a domU ?
> > > A driver assumes a (physical) device to be in the DomU, at which point it
> > > is CONFIG_PTDOM_MAX_ORDER which applies (PT standing for pass-through).
> > >
> > > > > However - does the driver really need this big a contiguous chunk? It
> > > > > would seem far more desirable to me to break that up some, if possible.
> > > > Since this works on bare metal I'm afraid the driver maintainer (mpt
> > > > fusion driver) will just tell me to fix Xen.
> > > Well. The bigger such allocations, the larger the risk that on systems
> > > that have been up for a while such allocations can't be fulfilled anymore
> > > even in the bare metal case.
> > Yes. I don't think we should just work around this issue without having
> > even tried to get the driver fixed. In case they refuse to change it, we
> > can still increase MAX_CONTIG_ORDER.
> Thanks for the feedback. I'll try to have a look at the driver if I have
> time to do so.
Another thought would be to change the generic DMA allocation to not require
alignment based on the rounded up size, but on the largest power-of-2 chunk
fitting into the requested size.

I don't see why a 2.3 MB memory allocation would need to be 4 MB aligned. It
should be perfectly fine to align it to 2 MB only.


Juergen

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