Re: [PATCH for-4.18 v3] x86/pvh: fix identity mapping of low 1MB

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.10.2023 10:29, Roger Pau Monne wrote:
> The mapping of memory regions below the 1MB mark was all done by the PVH dom0
> builder code, causing the region to be avoided by the arch specific IOMMU
> hardware domain initialization code.  That lead to the IOMMU being enabled
> without reserved regions in the low 1MB identity mapped in the p2m for PVH
> hardware domains.  Firmware which happens to be missing RMRR/IVMD ranges
> describing E820 reserved regions in the low 1MB would transiently trigger 
> IOMMU
> faults until the p2m is populated by the PVH dom0 builder:
> 
> AMD-Vi: IO_PAGE_FAULT: 0000:00:13.1 d0 addr 00000000000eb380 flags 0x20 RW
> AMD-Vi: IO_PAGE_FAULT: 0000:00:13.1 d0 addr 00000000000eb340 flags 0
> AMD-Vi: IO_PAGE_FAULT: 0000:00:13.2 d0 addr 00000000000ea1c0 flags 0
> AMD-Vi: IO_PAGE_FAULT: 0000:00:14.5 d0 addr 00000000000eb480 flags 0x20 RW
> AMD-Vi: IO_PAGE_FAULT: 0000:00:12.0 d0 addr 00000000000eb080 flags 0x20 RW
> AMD-Vi: IO_PAGE_FAULT: 0000:00:14.5 d0 addr 00000000000eb400 flags 0
> AMD-Vi: IO_PAGE_FAULT: 0000:00:12.0 d0 addr 00000000000eb040 flags 0
> 
> Those errors have been observed on the osstest pinot{0,1} boxes (AMD Fam15h
> Opteron(tm) Processor 3350 HE).
> 
> Rely on the IOMMU arch init code to create any identity mappings for reserved
> regions in the low 1MB range (like it already does for reserved regions
> elsewhere), and leave the mapping of any holes to be performed by the dom0
> builder code.
> 
> Fixes: 6b4f6a31ace1 ('x86/PVH: de-duplicate mappings for first Mb of Dom0 
> memory')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one suggestion:

> --- a/xen/arch/x86/hvm/dom0_build.c
> +++ b/xen/arch/x86/hvm/dom0_build.c
> @@ -449,7 +449,7 @@ static int __init pvh_populate_p2m(struct domain *d)
>          }
>      }
>  
> -    /* Non-RAM regions of space below 1MB get identity mapped. */
> +    /* Identity map everything below 1MB that's not already mapped. */
>      for ( i = rc = 0; i < MB1_PAGES; ++i )
>      {
>          p2m_type_t p2mt;
> @@ -459,8 +459,8 @@ static int __init pvh_populate_p2m(struct domain *d)
>              rc = set_mmio_p2m_entry(d, _gfn(i), _mfn(i), PAGE_ORDER_4K);
>          else
>              /*
> -             * If the p2m entry is already set it must belong to a RMRR and
> -             * already be identity mapped, or be a RAM region.
> +             * If the p2m entry is already set it must belong to a RMRR/IVMD 
> or
> +             * reserved region and be identity mapped, or else be a RAM 
> region.
>               */
>              ASSERT(p2mt == p2m_ram_rw || mfn_eq(mfn, _mfn(i)));

Would you mind wording the comment slightly differently, e.g.

"If the p2m entry is already set it must belong to a reserved region
 (e.g. RMRR/IVMD) and be identity mapped, or else be a RAM region."

This is because such RMRR/IVMD regions are required to be in reserved
ranges anyway.

Jan