[PATCH RFC 0/6] i386/pc: Fix creation of >= 1Tb guests on AMD systems with IOMMU

[Joao Martins]

Hey,

This series lets Qemu properly spawn i386 guests with >= 1Tb with VFIO, 
particularly
when running on AMD systems with an IOMMU.

Since Linux v5.4, VFIO validates whether the IOVA in DMA_MAP ioctl is valid and 
it
will return -EINVAL on those cases. On x86, Intel hosts aren't particularly
affected by this extra validation. But AMD systems with IOMMU have a hole in
the 1TB boundary which is *reserved* for HyperTransport I/O addresses located
here  FD_0000_0000h - FF_FFFF_FFFFh. See IOMMU manual [1], specifically
section '2.1.2 IOMMU Logical Topology', Table 3 on what those addresses mean.

VFIO DMA_MAP calls in this IOVA address range fall through this check and hence 
return
 -EINVAL, consequently failing the creation the guests bigger than 1010G. 
Example
of the failure:

qemu-system-x86_64: -device vfio-pci,host=0000:41:10.1,bootindex=-1: 
VFIO_MAP_DMA: -22
qemu-system-x86_64: -device vfio-pci,host=0000:41:10.1,bootindex=-1: vfio 
0000:41:10.1: 
        failed to setup container for group 258: memory listener initialization 
failed:
                Region pc.ram: vfio_dma_map(0x55ba53e7a9d0, 0x100000000, 
0xff30000000, 0x7ed243e00000) = -22 (Invalid argument)

Prior to v5.4, we could map using these IOVAs *but* that's still not the right 
thing
to do and could trigger certain IOMMU events (e.g. INVALID_DEVICE_REQUEST), or
spurious guest VF failures from the resultant IOMMU target abort (see Errata 
1155[2])
as documented on the links down below.

This series tries to address that by dealing with this AMD-specific 1Tb hole,
similarly to how we deal with the 4G hole today in x86 in general. It is 
splitted
as following:

* patch 1: initialize the valid IOVA ranges above 4G, adding an iterator
           which gets used too in other parts of pc/acpi besides MR creation. 
The
           allowed IOVA *only* changes if it's an AMD host, so no change for
           Intel. We walk the allowed ranges for memory above 4G, and
           add a E820_RESERVED type everytime we find a hole (which is at the
           1TB boundary).
           
           NOTE: For purposes of this RFC, I rely on cpuid in hw/i386/pc.c but I
           understand that it doesn't cover the non-x86 host case running TCG.

           Additionally, an alternative to hardcoded ranges as we do today,
           VFIO could advertise the platform valid IOVA ranges without 
necessarily
           requiring to have a PCI device added in the vfio container. That 
would
           fetching the valid IOVA ranges from VFIO, rather than hardcoded IOVA
           ranges as we do today. But sadly, wouldn't work for older 
hypervisors.

* patch 2 - 5: cover the remaining parts of the surrounding the mem map, 
particularly
               ACPI SRAT ranges, CMOS, hotplug as well as the PCI 64-bit hole.

* patch 6: Add a machine property which is disabled for older machine types 
(<=6.0)
           to keep things as is.

The 'consequence' of this approach is that we may need more than the default
phys-bits e.g. a guest with 1024G, will have ~13G be put after the 1TB
address, consequently needing 41 phys-bits as opposed to the default of 40.
I can't figure a reasonable way to establish the required phys-bits we
need for the memory map in a dynamic way, especially considering that
today there's already a precedent to depend on the user to pick the right value
of phys-bits (regardless of this series).

Additionally, the reserved region is always added regardless of whether we have
VFIO devices to cover the VFIO device hotplug case.

Other options considered:

a) Consider the reserved range part of RAM, and just marking it as
E820_RESERVED without SPA allocated for it. So a -m 1024G guest would
only allocate 1010G of RAM and the remaining would be marked reserved.
This is not how what we do today for the 4G hole i.e. the RAM
actually allocated is the value specified by the user and thus RAM available
to the guest (IIUC).

b) Avoid VFIO DMA_MAP ioctl() calls to the reserved range. Similar to a) but 
done at a
later stage when RAM mrs are already allocated at the invalid GPAs. Albeit that
alone wouldn't fix the way MRs are laid out which is where fundamentally the
problem is.

The proposed approach in this series works regardless of the kernel, and
relevant for old and new Qemu.

Open to alternatives/comments/suggestions that I should pursue instead.

        Joao

[1] https://www.amd.com/system/files/TechDocs/48882_IOMMU.pdf
[2] https://developer.amd.com/wp-content/resources/56323-PUB_0.78.pdf

Joao Martins (6):
  i386/pc: Account IOVA reserved ranges above 4G boundary
  i386/pc: Round up the hotpluggable memory within valid IOVA ranges
  pc/cmos: Adjust CMOS above 4G memory size according to 1Tb boundary
  i386/pc: Keep PCI 64-bit hole within usable IOVA space
  i386/acpi: Fix SRAT ranges in accordance to usable IOVA
  i386/pc: Add a machine property for AMD-only enforcing of valid IOVAs

 hw/i386/acpi-build.c  |  22 ++++-
 hw/i386/pc.c          | 206 +++++++++++++++++++++++++++++++++++++++---
 hw/i386/pc_piix.c     |   2 +
 hw/i386/pc_q35.c      |   2 +
 hw/pci-host/i440fx.c  |   4 +-
 hw/pci-host/q35.c     |   4 +-
 include/hw/i386/pc.h  |  62 ++++++++++++-
 include/hw/i386/x86.h |   4 +
 target/i386/cpu.h     |   3 +
 9 files changed, 288 insertions(+), 21 deletions(-)

-- 
2.17.1