Re: [PATCH v2 2/2] x86/xen: allow larger contiguous memory regions in PV guests

[Jan Beulich <jbeulich@suse.com>]

On 13.02.2025 09:55, Juergen Gross wrote:
> Today a PV guest (including dom0) can create 2MB contiguous memory
> regions for DMA buffers at max. This has led to problems at least
> with the megaraid_sas driver, which wants to allocate a 2.3MB DMA
> buffer.
> 
> The limiting factor is the frame array used to do the hypercall for
> making the memory contiguous, which has 512 entries and is just a
> static array in mmu_pv.c.
> 
> In order to not waste memory for non-PV guests, put the initial
> frame array into .init.data section and dynamically allocate an array
> from the .init_after_bootmem hook of PV guests.
> 
> In case a contiguous memory area larger than the initially supported
> 2MB is requested, allocate a larger buffer for the frame list. Note
> that such an allocation is tried only after memory management has been
> initialized properly, which is tested via a flag being set in the
> .init_after_bootmem hook.
> 
> Fixes: 9f40ec84a797 ("xen/swiotlb: add alignment check for dma buffers")
> Signed-off-by: Juergen Gross <jgross@suse.com>
> Tested-by: Alan Robinson <Alan.Robinson@fujitsu.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>