On 10/01/22 11:38 pm, Sean Christopherson wrote:
On Mon, Dec 20, 2021, Shivam Kumar wrote:
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9a2972fdae82..723f24909314 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10042,6 +10042,11 @@ static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
!vcpu->arch.apf.halted);
}
+static inline bool is_dirty_quota_full(struct kvm_vcpu *vcpu)
+{
+ return (vcpu->stat.generic.dirty_count >= vcpu->run->dirty_quota);
+}
+
static int vcpu_run(struct kvm_vcpu *vcpu)
{
int r;
@@ -10079,6 +10084,18 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
return r;
vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
}
+
+ /*
+ * Exit to userspace when dirty quota is full (if dirty quota
+ * throttling is enabled, i.e. dirty quota is non-zero).
+ */
+ if (vcpu->run->dirty_quota > 0 && is_dirty_quota_full(vcpu)) {
Kernel style is to omit the "> 0" when checking for non-zero. It matters here
because the "> 0" suggests dirty_quota can be negative, which it can't.
To allow userspace to modify dirty_quota on the fly, run->dirty_quota should be
READ_ONCE() with the result used for both the !0 and >= checks. And then also
capture the effective dirty_quota in the exit union struct (free from a memory
perspective because the exit union is padded to 256 bytes). That way if userspace
wants to modify the dirty_quota while the vCPU running it will get coherent data
even though the behavior is somewhat non-deterministic.
And then to simplify the code and also make this logic reusable for other
architectures, move it all into the helper and put the helper in kvm_host.h.
For other architectures, unless the arch maintainers explicitly don't want to
support this, I would prefer we enable at least arm64 right away to prevent this
from becoming a de facto x86-only feature. s390 also appears to be easy to support.
I almost suggested moving the check to generic code, but then I looked at MIPS
and PPC and lost all hope :-/
+ vcpu->run->exit_reason = KVM_EXIT_DIRTY_QUOTA_FULL;
--
I am not able to test this on arm64 and s390 as I don't have access to
arm64 and s390 hardware. Looking forward to your suggestions. Thank you!
