Re: [PATCH] tools/libs/guest: fix save and restore of pv domains after 32-bit de-support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.06.2021 11:04, Juergen Gross wrote:
> After 32-bit PV-guests have been security de-supported when not running
> under PV-shim, the hypervisor will no longer be configured to support
> those domains per default when not being built as PV-shim.
> 
> Unfortunately libxenguest will fail saving or restoring a PV domain
> due to this restriction, as it is trying to get the compat MFN list
> even for 64 bit guests.
> 
> Fix that by obtaining the compat MFN list only for 32-bit PV guests.
> 
> Fixes: 1a0f2fe2297d122a08fe ("SUPPORT.md: Un-shimmed 32-bit PV guests are no 
> longer supported")
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

As this will do for the single present consumer of the field
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Nevertheless I wonder ...

> --- a/tools/libs/guest/xg_sr_common_x86_pv.c
> +++ b/tools/libs/guest/xg_sr_common_x86_pv.c
> @@ -154,6 +154,7 @@ int x86_pv_map_m2p(struct xc_sr_context *ctx)
>      ctx->x86.pv.compat_m2p_mfn0 = entries[0].mfn;
>  #else
>      /* 64 bit toolstacks need to ask Xen specially for it */
> +    if ( ctx->x86.pv.levels == 3 )
>      {
>          struct xen_machphys_mfn_list xmml = {
>              .max_extents = 1,

... whether the field wouldn't better get set to an always-invalid
value in all other cases (either by pre-setting or by adding an
"else" here), e.g. ~0.

Jan