Re: [Xen-devel] [PATCH v2 05/10] libxl/xl: add memory policy option to iomem

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 30/04/2019 22:02, Stefano Stabellini wrote:
> Add a new memory policy option for the iomem parameter.
> Possible values are:
> - arm_devmem, device nGRE, the default on ARM
> - arm_memory, WB cachable memory
> - x86_uc: uncachable memory, the default on x86
>
> Store the parameter in a new field in libxl_iomem_range.
>
> Pass the memory policy option to xc_domain_mem_map_policy.
>
> Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> CC: ian.jackson@xxxxxxxxxxxxx
> CC: wei.liu2@xxxxxxxxxx
> ---
> Changes in v2:
> - add #define LIBXL_HAVE_MEMORY_POLICY
> - ability to part the memory policy parameter even if gfn is not passed
> - rename cache_policy to memory policy
> - rename MEMORY_POLICY_DEVMEM to MEMORY_POLICY_ARM_DEV_nGRE
> - rename MEMORY_POLICY_MEMORY to MEMORY_POLICY_ARM_MEM_WB
> - rename memory to arm_memory and devmem to arm_devmem
> - expand the non-security support status to non device passthrough iomem
>    configurations
> - rename iomem options
> - add x86 specific iomem option
> ---
>   SUPPORT.md                  |  2 +-
>   docs/man/xl.cfg.5.pod.in    |  7 ++++++-
>   tools/libxl/libxl.h         |  5 +++++
>   tools/libxl/libxl_create.c  | 21 +++++++++++++++++++--
>   tools/libxl/libxl_types.idl |  9 +++++++++
>   tools/xl/xl_parse.c         | 22 +++++++++++++++++++++-
>   6 files changed, 61 insertions(+), 5 deletions(-)
>
> diff --git a/SUPPORT.md b/SUPPORT.md
> index e4fb15b..f29a299 100644
> --- a/SUPPORT.md
> +++ b/SUPPORT.md
> @@ -649,7 +649,7 @@ to be used in addition to QEMU.
>   
>   	Status: Experimental
>   
> -### ARM/Non-PCI device passthrough
> +### ARM/Non-PCI device passthrough and other iomem configurations
I am not sure why iomem is added here? Also what was the security support before 
hand? Was it supported?
>   
>       Status: Supported, not security supported
>   
> diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
> index c7d70e6..c85857e 100644
> --- a/docs/man/xl.cfg.5.pod.in
> +++ b/docs/man/xl.cfg.5.pod.in
> @@ -1222,7 +1222,7 @@ is given in hexadecimal format and may either be a range, e.g. 
> C<2f8-2ff>
>   It is recommended to only use this option for trusted VMs under
>   administrator's control.
>   
> -=item B<iomem=[ "IOMEM_START,NUM_PAGES[@GFN]", "IOMEM_START,NUM_PAGES[@GFN]", ...]>
> +=item B<iomem=[ "IOMEM_START,NUM_PAGES[@GFN],MEMORY_POLICY", 
> "IOMEM_START,NUM_PAGES[@GFN][,MEMORY_POLICY]", ...]>
>   
>   Allow auto-translated domains to access specific hardware I/O memory pages.
>   
> @@ -1233,6 +1233,11 @@ B<GFN> is not specified, the mapping will be performed using B<IOMEM_START>
>   as a start in the guest's address space, therefore performing a 1:1 mapping
>   by default.
>   All of these values must be given in hexadecimal format.
> +B<MEMORY_POLICY> for ARM platforms:
> +  - "arm_devmem" for Device nGRE, the default on ARM
This does not match the current default. At the moment, it is Device nGnRE.

> +  - "arm_memory" for Outer Shareable Write-Back Cacheable Memory
The two names are quite confusing and will make quite difficult to introduce any 
new one. It also make little sense to use a different naming in xl and libxl. 
This only add an other level of confusion.
Overall, this is not enough for a user to understand the memory policy. As I 
pointed out before, this is not straight forward on Arm as the resulting memory 
attribute will be a combination of stage-2 and stage-1.
We need to explain the implication of using the memory and the consequence of 
misuse it. And particularly as this is not security supported so we don't end up 
to security support in the future something that don't work.
Cheers,

--
Julien Grall

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