RFC xen/arm: fix "xen_add_mach_to_phys_entry: cannot add" problem
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Cover letter [Xen-devel] [RFC PATCH 0/2] xen/arm: fix "xen_add_mach_to_phys_entry: cannot add" problem
Quoted 1 line · Stefano Stabellini
Hi all,
Hi Stefano,
Quoted 7 lines
Xen support in Linux for ARM and ARM64 suffers from lack of support for multiple mfn to pfn mappings: whenever a frontend grants the same page multiple times to the backend, the mfn to pfn accounting in
arch/arm/xen/p2m.cfails. The issue has become critical since v3.15, when xen-netfront/xen-netback switched from grant copies to grant mappings, therefore causing the issue to happen much more often.
Quoted 8 lines
Fixing the mfn to pfn accounting in
p2m.cis difficult and expensive, therefore we are looking for alternative solutions. One idea is avoiding mfn to pfn conversions altogether. The only code path that needs them is swiotlb-xen:unmap_page (andsingle_for_cpuandsingle_for_device).To avoid mfn to pfn conversions we rely on a second p2m mapping done by Xen, see the attached patch. In Linux we use it to perform the cache maintenance operations without mfns conversions.
I tried this series on midway with a FreeBSD guest. I got those errors in DOM0 log:
xen_add_phys_to_mach_entry: cannot add pfn=0x002d99330003cdc7 -> mfn=0x00000001002d9933: pfn=0x002d99330003e3ea -> mfn=0x00000001002d9933 already existsxen_add_phys_to_mach_entry: cannot add pfn=0x002d99330003d47a -> mfn=0x00000001002d9933: pfn=0x002d99330003e3ea -> mfn=0x00000001002d9933 already exists
It looks like you forgot to modify xen_add_phys_to_mach_entry.
Regards,
Signature · 1 line
Julien Grall