Re: [PATCH v2 2/2] x86/dom0: attempt to fixup p2m page-faults for PVH dom0

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Feb 18, 2025 at 03:35:04PM +0100, 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>
> ---
> Only slightly tested on my local PVH dom0 deployment.
> ---
> Changes since v1:
>  - Make the fixup function static.
>  - Print message in case mapping already exists.
> ---
>  CHANGELOG.md                           | 10 ++++
>  docs/misc/xen-command-line.pandoc      | 16 +++++-
>  xen/arch/x86/dom0_build.c              |  5 ++
>  xen/arch/x86/hvm/emulate.c             | 74 +++++++++++++++++++++++++-
>  xen/arch/x86/include/asm/hvm/emulate.h |  3 ++
>  5 files changed, 105 insertions(+), 3 deletions(-)
> 
> diff --git a/CHANGELOG.md b/CHANGELOG.md
> index 1de1d1eca17f..e5e6ab3a8902 100644
> --- a/CHANGELOG.md
> +++ b/CHANGELOG.md
> @@ -4,6 +4,16 @@ Notable changes to Xen will be documented in this file.
>  
>  The format is based on [Keep a 
> Changelog](https://keepachangelog.com/en/1.0.0/)
>  
> +## [4.21.0 
> UNRELEASED](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=staging)
>  - TBD
> +
> +### Changed
> +
> +### Added
> + - On x86:
> +   - Option to attempt to fixup p2m page-faults on PVH dom0.
> +
> +### Removed
> +
>  ## [4.20.0 
> UNRELEASED](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=staging)
>  - TBD
>  
>  ### Changed
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index 9bbd00baef91..83bb69cfb852 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -822,7 +822,8 @@ Specify the bit width of the DMA heap.
>  
>  ### dom0
>      = List of [ pv | pvh, shadow=<bool>, verbose=<bool>,
> -                cpuid-faulting=<bool>, msr-relaxed=<bool> ] (x86)
> +                cpuid-faulting=<bool>, msr-relaxed=<bool>,
> +                pf-fixup=<bool> ] (x86)
>  
>      = List of [ sve=<integer> ] (Arm64)
>  
> @@ -883,6 +884,19 @@ Controls for how dom0 is constructed on x86 systems.
>  
>      If using this option is necessary to fix an issue, please report a bug.
>  
> +*   The `pf-fixup` boolean is only applicable when using a PVH dom0 and
> +    defaults to false.

I'm considering whether the default should instead be true, so that
PVH is closer to what which regions a classic PV dom0 gets access to.

Thanks, Roger.