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qemu/machine: fix highmem_mmio_size doc and Makefile claim

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The highmem_mmio_size doc comment ended with a dangling "bytes." line;
fold the unit into the opening sentence, in virt.rs and the copy quoted
in ARCHITECTURE.md.

ARCHITECTURE.md claimed the Makefile already defaults CPUFEATURES and
TDXCPUFEATURES to include host-phys-bits=on; the repo defaults are
still pmu=off only.  State the actual situation: the flag must be set
per configuration file until the Makefile defaults are extended.

Assisted-by: Claude <noreply@anthropic.com>
Signed-off-by: Zvonko Kaiser <zkaiser@nvidia.com>
This commit is contained in:
Zvonko Kaiser
2026-07-17 16:14:52 +00:00
parent dc5fdc0c0a
commit e1698f08e8
2 changed files with 10 additions and 10 deletions
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14
src/runtime-rs/crates/hypervisor/src/qemu/ARCHITECTURE.md
View File

@@ -139,9 +139,9 @@ pub struct Virt {
pub base: BaseMachine,
pub gic_version: Option<u8>,
pub ras: bool,
/// Required for Grace GPU passthrough; must be a power of 2.
/// 4T for GH200/GB200 (≤4 GPUs), 8T for GB300 NVL72 (4 GPUs).
/// bytes.
/// Highmem MMIO window size in bytes; must be a power of 2.
/// Required for Grace GPU passthrough: 4T for GH200/GB200 (≤4 GPUs),
/// 8T for GB300 NVL72 (4 GPUs).
pub highmem_mmio_size: Option<u64>,
// pub runtime: RuntimeFeatures, -- Phase 3+
}
@@ -840,10 +840,10 @@ three GPUs (384 GiB) fit within 1 TiB, but four GPUs (512 GiB) do not, causing
the fourth GPU to fail PCIe BAR assignment at VM boot.
**Fix:** set `cpu_features = "pmu=off,host-phys-bits=on"` in
`configuration-qemu-nvidia-gpu*.toml.in`. The `Makefile` now defaults
`CPUFEATURES` and `TDXCPUFEATURES` to `pmu=off,host-phys-bits=on` for `amd64`
builds so operator installs that regenerate configs from source pick up the fix;
existing installed configs must be updated manually.
`configuration-qemu-nvidia-gpu*.toml.in`. The `Makefile` defaults
(`CPUFEATURES`, `TDXCPUFEATURES`) still carry only `pmu=off`; extending them
to include `host-phys-bits=on` for `amd64` builds is a separate pending
change, so today the fix must be applied per configuration file.
**Post-refactor:** the `Platform` Q35 builder should emit `host-phys-bits=on`
unconditionally for x86_64/KVM so the fix is durable even if the config file

6
src/runtime-rs/crates/hypervisor/src/qemu/machine/virt.rs
View File

@@ -8,8 +8,8 @@ pub(crate) struct Virt {
pub base: BaseMachine,
pub gic_version: Option<u8>,
pub ras: bool,
/// Required for Grace GPU passthrough; must be a power of 2.
/// 4T for GH200/GB200 with <=4 GPUs, 8T for GB300 NVL72 with 4 GPUs.
/// bytes.
/// Highmem MMIO window size in bytes; must be a power of 2.
/// Required for Grace GPU passthrough: 4T for GH200/GB200 with <=4
/// GPUs, 8T for GB300 NVL72 with 4 GPUs.
pub highmem_mmio_size: Option<u64>,
}