Re: [PATCH for-4.18 v2] x86/pvh: fix identity mapping of low 1MB

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Oct 16, 2023 at 03:32:54PM +0200, Jan Beulich wrote:
> On 13.10.2023 10:56, 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).
> > 
> > Mostly remove the special handling of the low 1MB done by the PVH dom0 builder,
> > leaving just the data copy between RAM regions.  Otherwise rely on the IOMMU
> > arch init code to create any identity mappings for reserved regions in that
> > range (like it already does for reserved regions elsewhere).
> > 
> > Note there's a small difference in behavior, as holes in the low 1MB will no
> > longer be identity mapped to the p2m.
> 
> I certainly like the simplification, but I'm concerned by this: The BDA
> is not normally reserved, yet may want accessing by Dom0 (to see the real
> machine contents). We do access that first page of memory ourselves, so
> I expect OSes may do so as well (even if the specific aspect I'm thinking
> of - the warm/cold reboot field - is under Xen's control).

The BDA on the systems I've checked falls into a RAM area on the
memory map, but if you think it can be problematic I could arrange for
arch_iommu_hwdom_init() to also identity map holes in the low 1MB.

Keep in mind this is only for PVH, it won't affect PV.

Thanks, Roger.