Re: [PATCH kernel v3 2/3] KVM: SEV: Enable data breakpoints in SEV-ES
From: Alexey Kardashevskiy
Date: Thu Feb 02 2023 - 22:37:48 EST
On 01/02/2023 13:18, Sean Christopherson wrote:
On Fri, Jan 20, 2023, Alexey Kardashevskiy wrote:
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 4826e6cc611b..61f2cad1cbaf 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -389,6 +389,8 @@ static inline bool vmcb12_is_intercept(struct vmcb_ctrl_area_cached *control, u3
return test_bit(bit, (unsigned long *)&control->intercepts);
}
+extern bool sev_es_is_debug_swap_enabled(void);
+
static inline void set_dr_intercepts(struct vcpu_svm *svm)
{
struct vmcb *vmcb = svm->vmcb01.ptr;
@@ -410,8 +412,10 @@ static inline void set_dr_intercepts(struct vcpu_svm *svm)
vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_WRITE);
}
- vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
- vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
+ if (!sev_es_guest(svm->vcpu.kvm) || !sev_es_is_debug_swap_enabled()) {
Looking below, doesn't this do the wrong thing if set_dr_intercepts() is called
before SVM_SEV_FEAT_DEBUG_SWAP is set? I.e. when this is called before LAUNCH_UPDATE?
Seems like this should check SVM_SEV_FEAT_DEBUG_SWAP in sev_features regardless
of when SVM_SEV_FEAT_DEBUG_SWAP is set.
And if KVM checks sev_features, then I _think_ we can avoid having to expose
sev_es_debug_swap_enabled to svm.{c,h} (though why on earth {set,clr}_dr_intercepts()
is in svm.h is another question for the future).
883b0a91f41a ("KVM: SVM: Move Nested SVM Implementation to nested.c")
did that. Makes sense for things like vmcb_set_intercept() but
{set,clr}_dr_intercepts() are still only called from svm.c so I'll move
them there (btw do I need a separate patch for that? usually yes)
Follow-up question: does KVM _have_ to wait until KVM_SEV_LAUNCH_UPDATE_VMSA to
set the flag?
Nope. Will repost soon as a reply to this mail.
+ vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
+ vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
+ }
recalc_intercepts(svm);
}
@@ -422,8 +426,12 @@ static inline void clr_dr_intercepts(struct vcpu_svm *svm)
vmcb->control.intercepts[INTERCEPT_DR] = 0;
- /* DR7 access must remain intercepted for an SEV-ES guest */
- if (sev_es_guest(svm->vcpu.kvm)) {
+ /*
+ * DR7 access must remain intercepted for an SEV-ES guest unless DebugSwap
+ * (depends on NO_NESTED_DATA_BP) is enabled as otherwise a VM writing to DR7
+ * from the #DB handler may trigger infinite loop of #DB's.
+ */
+ if (sev_es_guest(svm->vcpu.kvm) && !sev_es_is_debug_swap_enabled()) {
vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
}
@@ -52,11 +53,21 @@ module_param_named(sev, sev_enabled, bool, 0444);
/* enable/disable SEV-ES support */
static bool sev_es_enabled = true;
module_param_named(sev_es, sev_es_enabled, bool, 0444);
+
+/* enable/disable SEV-ES DebugSwap support */
+static bool sev_es_debug_swap_enabled = true;
+module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0644);
Module param needs 0444 permissions, i.e. shouldn't be writable after KVM is
loaded. Though I don't know that providing a module param is warranted in this
case.
KVM provides module params for SEV and SEV-ES because there are legitimate
reasons to turn them off, but at a glance, I don't see why we'd want that for this
feature.
/me confused. You suggested this in the first place for (I think) for
the case if the feature is found to be broken later on so admins can
disable it.
And I was using it to verify "x86/debug: Fix stack recursion caused by
DR7 accesses" which is convenient but it is a minor thing.
#else
#define sev_enabled false
#define sev_es_enabled false
+#define sev_es_debug_swap false
This needs to be sev_es_debug_swap_enabled, otherwise things fall apart with
CONFIG_KVM_AMD_SEV=n.
arch/x86/kvm/svm/sev.c: In function ‘sev_es_is_debug_swap_enabled’:
arch/x86/kvm/svm/sev.c:69:16: error: ‘sev_es_debug_swap_enabled’ undeclared (first use in this function); did you mean ‘sev_es_is_debug_swap_enabled’?
69 | return sev_es_debug_swap_enabled;
| ^~~~~~~~~~~~~~~~~~~~~~~~~
| sev_es_is_debug_swap_enabled
#endif /* CONFIG_KVM_AMD_SEV */
+bool sev_es_is_debug_swap_enabled(void)
+{
+ return sev_es_debug_swap_enabled;
+}
...
@@ -604,6 +615,9 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
save->xss = svm->vcpu.arch.ia32_xss;
save->dr6 = svm->vcpu.arch.dr6;
+ if (sev_es_is_debug_swap_enabled())
+ save->sev_features |= SVM_SEV_FEAT_DEBUG_SWAP;
+
pr_debug("Virtual Machine Save Area (VMSA):\n");
print_hex_dump_debug("", DUMP_PREFIX_NONE, 16, 1, save, sizeof(*save), false);
@@ -2249,6 +2263,9 @@ void __init sev_hardware_setup(void)
out:
sev_enabled = sev_supported;
sev_es_enabled = sev_es_supported;
+ if (sev_es_debug_swap_enabled)
+ sev_es_debug_swap_enabled = sev_es_enabled &&
+ cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP);
Slight preference for:
if (!sev_es_enabled || !cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP))
sev_es_debug_swap_enabled = false;
KVM does short-circuit some checks on module param values, but usually only to
avoid additional setup.
#endif
}
@@ -3027,6 +3044,18 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
/* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */
hostsa->xss = host_xss;
+
+ /* The DebugSwap SEV feature does Type B swaps of DR[0-3] */
+ if (sev_es_is_debug_swap_enabled()) {
+ hostsa->dr0 = native_get_debugreg(0);
+ hostsa->dr1 = native_get_debugreg(1);
+ hostsa->dr2 = native_get_debugreg(2);
+ hostsa->dr3 = native_get_debugreg(3);
+ hostsa->dr0_addr_mask = amd_get_dr_addr_mask(0);
+ hostsa->dr1_addr_mask = amd_get_dr_addr_mask(1);
+ hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);
+ hostsa->dr3_addr_mask = amd_get_dr_addr_mask(3);
+ }
}
void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 60c7c880266b..6c54a3c9d442 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1190,7 +1190,8 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
set_exception_intercept(svm, UD_VECTOR);
set_exception_intercept(svm, MC_VECTOR);
set_exception_intercept(svm, AC_VECTOR);
- set_exception_intercept(svm, DB_VECTOR);
+ if (!sev_es_is_debug_swap_enabled())
+ set_exception_intercept(svm, DB_VECTOR);
This is wrong. KVM needs to intercept #DBs when debugging non-SEV-ES VMs.
Sorry, not following. The #DB intercept for non-SEV-ES is enabled here.
Thanks,
This _could_ be tied to X86_FEATURE_NO_NESTED_DATA_BP, but the KVM would need to
toggle the intercept depending on whether or not userspace wants to debug the
guest.
Similar to the DR7 interception, can this check sev_features directly?
--
Alexey