On 5/8/2024 1:27 AM, Sean Christopherson wrote:
On Mon, May 06, 2024, Weijiang Yang wrote:
On 5/2/2024 6:40 AM, Sean Christopherson wrote:
On Sun, Feb 18, 2024, Yang Weijiang wrote:
[...]
For the latter, IIUC KVM still needs to expose the index within the synthetic
namespace so that userspace can read/write the intended MSRs, of course not
expose the synthetic MSR index via existing uAPI, But you said the "index"
exposed to userspace can simply be '0' in this case, then how to distinguish
the synthetic MSRs in userspace and KVM? And how userspace can be aware of
the synthetic MSR index allocation in KVM?
The idea is to have a synthetic index that is exposed to userspace, and a separate
KVM-internal index for emulating accesses. The value that is exposed to userspace
can start at 0 and be a simple incrementing value as we add synthetic MSRs, as the
.type == SYNTHETIC makes it impossible for the value to collide with a "real" MSR.
Translating to a KVM-internal index is a hack to avoid having to plumb a 64-bit
index into all the MSR code. We could do that, i.e. pass the full kvm_x86_reg_id
into the MSR helpers, but I'm not convinced it'd be worth the churn. That said,
I'm not opposed to the idea either, if others prefer that approach.
E.g.
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 738c449e4f9e..21152796238a 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -420,6 +420,8 @@ struct kvm_x86_reg_id {
__u16 rsvd16;
};
+#define MSR_KVM_GUEST_SSP 0
+
#define KVM_SYNC_X86_REGS (1UL << 0)
#define KVM_SYNC_X86_SREGS (1UL << 1)
#define KVM_SYNC_X86_EVENTS (1UL << 2)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f45cdd9d8c1f..1a9e1e0c9f49 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5990,6 +5990,19 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
}
}
+static int kvm_translate_synthetic_msr(u32 *index)
+{
+ switch (*index) {
+ case MSR_KVM_GUEST_SSP:
+ *index = MSR_KVM_INTERNAL_GUEST_SSP;
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
long kvm_arch_vcpu_ioctl(struct file *filp,
unsigned int ioctl, unsigned long arg)
{
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index cc585051d24b..3b5a038f5260 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -49,6 +49,15 @@ void kvm_spurious_fault(void);
#define KVM_FIRST_EMULATED_VMX_MSR MSR_IA32_VMX_BASIC
#define KVM_LAST_EMULATED_VMX_MSR MSR_IA32_VMX_VMFUNC
+/*
+ * KVM's internal, non-ABI indices for synthetic MSRs. The values themselves
+ * are arbitrary and have no meaning, the only requirement is that they don't
+ * conflict with "real" MSRs that KVM supports. Use values at the uppper end
+ * of KVM's reserved paravirtual MSR range to minimize churn, i.e. these values
+ * will be usable until KVM exhausts its supply of paravirtual MSR indices.
+ */
+#define MSR_KVM_INTERNAL_GUEST_SSP 0x4b564dff
+
#define KVM_DEFAULT_PLE_GAP 128
#define KVM_VMX_DEFAULT_PLE_WINDOW 4096
#define KVM_DEFAULT_PLE_WINDOW_GROW 2
OK, I'll post an RFC patch for this change, thanks a lot!
