Re: [PATCH] x86/PVH: expose OEMx ACPI tables to Dom0

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

On Wed, Mar 26, 2025 at 09:45:09AM +0100, Jan Beulich wrote:
> What they contain we don't know, but we can't sensibly hide them. On my
> Skylake system OEM1 (with a description of "INTEL  CPU EIST") is what
> contains all the _PCT, _PPC, and _PSS methods, i.e. about everything
> needed for cpufreq. (_PSD interestingly are in an SSDT there.)
> 
> Further OEM2 there has a description of "INTEL  CPU  HWP", while OEM4
> has "INTEL  CPU  CST". Pretty clearly all three need exposing for
> cpufreq and cpuidle to work.
> 
> Fixes: 8b1a5268daf0 ("pvh/dom0: whitelist PVH Dom0 ACPI tables")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> Sadly the commit above says nothing at all about the criteria used by
> which tables would want to be whitelisted.

I think at that point it was mostly an allow list that enabled me to
boot PVH on the systems I was testing.  I don't think it was intended
to be complete, but rather something that we would expand as needed.
There where many and bigger missing pieces of PVH dom0 when that was
committed.

> Further tables on said system
> which weren't blacklisted prior to that commit, yet which also aren't
> whitelisted are DBGP, DBG2, FIDT, LPIT, MIGT, MSCT, NITR, PCCT, RASF,
> SVOS, UEFI, WDDT, and WSMT. Even without diving into the details of any
> of them it seems pretty clear to me that at least some would want
> whitelisting, too.

I cannot find any reference about: FIDT, MIGT, NITR, SVOS and WDDT in
the ACPI spec.

The MSCT I think we don't want to expose, as it's related to topology
data.

Regarding RASF I would be slightly worried about the patrol scrub
feature.  The memory map exposed to dom0 will be different from the
native one, and there's also the interposed p2m.

The rest are likely fine to expose.

> 
> --- a/xen/arch/x86/hvm/dom0_build.c
> +++ b/xen/arch/x86/hvm/dom0_build.c
> @@ -1010,12 +1010,20 @@ static bool __init pvh_acpi_table_allowe
>              return true;
>          else
>          {
> +    skip:
>              printk("Skipping table %.4s in non-ACPI non-reserved region\n",
>                     sig);
>              return false;
>          }
>      }
>  
> +    if ( !strncmp(sig, "OEM", 3) )
> +    {
> +        if ( acpi_memory_banned(address, size) )
> +            goto skip;
> +        return true;
> +    }

I may have put this ahead of the loop, so that the goto label doesn't
go backwards (which always feels weird to me).

> +

I wonder if additionally we should print tables filtered to dom0 here:

if ( opt_dom0_verbose )
    printk("Hidden ACPI Table %.4s\n", sig);

So that it's more obvious which tables are not exposed.

Thanks, Roger.