Le 13/04/2021 à 00:26, Fabiano Rosas a écrit :
The KVM_CAP_MAX_VCPU_ID capability was added by commit 0b1b1dfd52a6
("kvm: introduce KVM_MAX_VCPU_ID") to allow for vcpu ids larger than
KVM_MAX_VCPU in powerpc.
For a P9 host we depend on the guest VSMT value to know what is the
maximum number of vcpu id we can support:
kvmppc_core_vcpu_create_hv:
(...)
if (cpu_has_feature(CPU_FTR_ARCH_300)) {
--> if (id >= (KVM_MAX_VCPUS * kvm->arch.emul_smt_mode)) {
pr_devel("KVM: VCPU ID too high\n");
core = KVM_MAX_VCORES;
} else {
BUG_ON(kvm->arch.smt_mode != 1);
core = kvmppc_pack_vcpu_id(kvm, id);
}
} else {
core = id / kvm->arch.smt_mode;
}
which means that the value being returned by the capability today for
a given guest is potentially way larger than what we actually support:
\#define KVM_MAX_VCPU_ID (MAX_SMT_THREADS * KVM_MAX_VCORES)
If the capability is queried before userspace enables the
KVM_CAP_PPC_SMT ioctl there is not much we can do, but if the emulated
smt mode is already known we could provide a more accurate value.
The only practical effect of this change today is to make the
kvm_create_max_vcpus test pass for powerpc. The QEMU spapr machine has
a lower max vcpu than what KVM allows so even KVM_MAX_VCPU is not
reached.
Signed-off-by: Fabiano Rosas <farosas@xxxxxxxxxxxxx>
This patch won't apply after commit a1c42ddedf35 ("kvm: rename
KVM_MAX_VCPU_ID to KVM_MAX_VCPU_IDS")
Feel free to resubmit if still applicable.
Christsophe
---
I see that for ppc, QEMU uses the capability after enabling
KVM_CAP_PPC_SMT, so we could change QEMU to issue the check extension
on the vm fd so that it would get the more accurate value.
---
arch/powerpc/kvm/powerpc.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index a2a68a958fa0..95c9f47cc1b3 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -649,7 +649,10 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
r = KVM_MAX_VCPUS;
break;
case KVM_CAP_MAX_VCPU_ID:
- r = KVM_MAX_VCPU_ID;
+ if (hv_enabled && cpu_has_feature(CPU_FTR_ARCH_300))
+ r = KVM_MAX_VCPUS * ((kvm) ? kvm->arch.emul_smt_mode : 1);
+ else
+ r = KVM_MAX_VCPU_ID;
break;
#ifdef CONFIG_PPC_BOOK3S_64
case KVM_CAP_PPC_GET_SMMU_INFO:
