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Re: [PATCH v2 2/2] x86/dom0: attempt to fixup p2m page-faults for PVH dom0
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 18 Feb 2025 16:57:12 +0100
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- Cc: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Community Manager <community.manager@xxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 18 Feb 2025 15:57:24 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 18.02.2025 15:35, Roger Pau Monne wrote:
> When building a PVH dom0 Xen attempts to map all (relevant) MMIO regions
> into the p2m for dom0 access. However the information Xen has about the
> host memory map is limited. Xen doesn't have access to any resources
> described in ACPI dynamic tables, and hence the p2m mappings provided might
> not be complete.
>
> PV doesn't suffer from this issue because a PV dom0 is capable of mapping
> into it's page-tables any address not explicitly banned in d->iomem_caps.
>
> Introduce a new command line options that allows Xen to attempt to fixup
> the p2m page-faults, by creating p2m identity maps in response to p2m
> page-faults.
>
> This is aimed as a workaround to small ACPI regions Xen doesn't know about.
> Note that missing large MMIO regions mapped in this way will lead to
> slowness due to the VM exit processing, plus the mappings will always use
> small pages.
>
> The ultimate aim is to attempt to bring better parity with a classic PV
> dom0.
>
> Note such fixup rely on the CPU doing the access to the unpopulated
> address. If the access is attempted from a device instead there's no
> possible way to fixup, as IOMMU page-fault are asynchronous.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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