Re: [PATCH] KVM: nVMX: Remove non-functional "support" for CR3 target values
From: Krish Sadhukhan
Date: Thu Apr 16 2020 - 21:37:38 EST
On 4/15/20 5:07 PM, Sean Christopherson wrote:
Remove all references to cr3_target_value[0-3] and replace the fields
in vmcs12 with "dead_space" to preserve the vmcs12 layout. KVM doesn't
support emulating CR3-target values, despite a variety of code that
implies otherwise, as KVM unconditionally reports '0' for the number of
supported CR3-target values.
This technically fixes a bug where KVM would incorrectly allow VMREAD
and VMWRITE to nonexistent fields, i.e. cr3_target_value[0-3]. Per
Intel's SDM, the number of supported CR3-target values reported in
VMX_MISC also enumerates the existence of the associated VMCS fields:
If a future implementation supports more than 4 CR3-target values, they
will be encoded consecutively following the 4 encodings given here.
Alternatively, the "bug" could be fixed by actually advertisting support
for 4 CR3-target values, but that'd likely just enable kvm-unit-tests
given that no one has complained about lack of support for going on ten
years, e.g. KVM, Xen and HyperV don't use CR3-target values.
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
Krish's patch to allow L1 to disable unrestricted guest got me poking
around the related code. That led me into nested_vmx_exit_handled_cr()
and handle_cr(). handle_cr() does WARN_ON_ONCE(enable_unrestricted_guest)
in the MOV-to-CR3 path, as KVM should never enable CR3-exiting for L1.
But the CR3-target matching nested_vmx_exit_handled_cr() meant that it
would be possible to trigger the warn by enabling CR3-exiting in L1 with
a valid list of CR3-targets. I got so far as to write a patch and a unit
test, and then I checked MSR_IA32_VMX_MISC under KVM...
AFAICT, properly emulating CR3-targets would be a trivial patch. But, I
don't think there would be any meaningful users, e.g. Xen is the one
hypervisor (that I am aware of) that runs with EPT disabled for certain
use cases, and even Xen doesn't bother with CR3-targets. On the other
hand, the odds of introducing a bug of one form or another seem high.
Amusing side note, Xen's only reference to vmcs.CR3_TARGET_VALUE* is in
its vmcs_dump_vcpu(), which explains why KVM's dump_vmcs() bothers to spit
out the unused CR3-target fields :-).
Documentation/virt/kvm/nested-vmx.rst | 5 +----
arch/x86/kvm/vmx/evmcs.c | 8 --------
arch/x86/kvm/vmx/nested.c | 17 -----------------
arch/x86/kvm/vmx/vmcs12.c | 4 ----
arch/x86/kvm/vmx/vmcs12.h | 10 ++--------
arch/x86/kvm/vmx/vmx.c | 9 ---------
6 files changed, 3 insertions(+), 50 deletions(-)
diff --git a/Documentation/virt/kvm/nested-vmx.rst b/Documentation/virt/kvm/nested-vmx.rst
index 592b0ab6970b..89851cbb7df9 100644
--- a/Documentation/virt/kvm/nested-vmx.rst
+++ b/Documentation/virt/kvm/nested-vmx.rst
@@ -116,10 +116,7 @@ struct shadow_vmcs is ever changed.
natural_width cr4_guest_host_mask;
natural_width cr0_read_shadow;
natural_width cr4_read_shadow;
- natural_width cr3_target_value0;
- natural_width cr3_target_value1;
- natural_width cr3_target_value2;
- natural_width cr3_target_value3;
+ natural_width dead_space[4]; /* Last remnants of cr3_target_value[0-3]. */
natural_width exit_qualification;
natural_width guest_linear_address;
natural_width guest_cr0;
diff --git a/arch/x86/kvm/vmx/evmcs.c b/arch/x86/kvm/vmx/evmcs.c
index 303813423c3e..73f3e07c1852 100644
--- a/arch/x86/kvm/vmx/evmcs.c
+++ b/arch/x86/kvm/vmx/evmcs.c
@@ -160,14 +160,6 @@ const struct evmcs_field vmcs_field_to_evmcs_1[] = {
HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL),
EVMCS1_FIELD(VM_ENTRY_MSR_LOAD_ADDR, vm_entry_msr_load_addr,
HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL),
- EVMCS1_FIELD(CR3_TARGET_VALUE0, cr3_target_value0,
- HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL),
- EVMCS1_FIELD(CR3_TARGET_VALUE1, cr3_target_value1,
- HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL),
- EVMCS1_FIELD(CR3_TARGET_VALUE2, cr3_target_value2,
- HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL),
- EVMCS1_FIELD(CR3_TARGET_VALUE3, cr3_target_value3,
- HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL),
/* 32 bit rw */
EVMCS1_FIELD(TPR_THRESHOLD, tpr_threshold,
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index aca57d8da400..aeb56b515c5c 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1785,10 +1785,6 @@ static int copy_enlightened_to_vmcs12(struct vcpu_vmx *vmx)
* vmcs12->vm_exit_msr_store_addr = evmcs->vm_exit_msr_store_addr;
* vmcs12->vm_exit_msr_load_addr = evmcs->vm_exit_msr_load_addr;
* vmcs12->vm_entry_msr_load_addr = evmcs->vm_entry_msr_load_addr;
- * vmcs12->cr3_target_value0 = evmcs->cr3_target_value0;
- * vmcs12->cr3_target_value1 = evmcs->cr3_target_value1;
- * vmcs12->cr3_target_value2 = evmcs->cr3_target_value2;
- * vmcs12->cr3_target_value3 = evmcs->cr3_target_value3;
* vmcs12->page_fault_error_code_mask =
* evmcs->page_fault_error_code_mask;
* vmcs12->page_fault_error_code_match =
@@ -1862,10 +1858,6 @@ static int copy_vmcs12_to_enlightened(struct vcpu_vmx *vmx)
* evmcs->vm_exit_msr_store_addr = vmcs12->vm_exit_msr_store_addr;
* evmcs->vm_exit_msr_load_addr = vmcs12->vm_exit_msr_load_addr;
* evmcs->vm_entry_msr_load_addr = vmcs12->vm_entry_msr_load_addr;
- * evmcs->cr3_target_value0 = vmcs12->cr3_target_value0;
- * evmcs->cr3_target_value1 = vmcs12->cr3_target_value1;
- * evmcs->cr3_target_value2 = vmcs12->cr3_target_value2;
- * evmcs->cr3_target_value3 = vmcs12->cr3_target_value3;
* evmcs->tpr_threshold = vmcs12->tpr_threshold;
* evmcs->virtual_processor_id = vmcs12->virtual_processor_id;
* evmcs->exception_bitmap = vmcs12->exception_bitmap;
@@ -5540,15 +5532,6 @@ static bool nested_vmx_exit_handled_cr(struct kvm_vcpu *vcpu,
return true;
break;
case 3:
- if ((vmcs12->cr3_target_count >= 1 &&
- vmcs12->cr3_target_value0 == val) ||
- (vmcs12->cr3_target_count >= 2 &&
- vmcs12->cr3_target_value1 == val) ||
- (vmcs12->cr3_target_count >= 3 &&
- vmcs12->cr3_target_value2 == val) ||
- (vmcs12->cr3_target_count >= 4 &&
- vmcs12->cr3_target_value3 == val))
- return false;
if (nested_cpu_has(vmcs12, CPU_BASED_CR3_LOAD_EXITING))
return true;
break;
diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c
index 53dfb401316d..c8e51c004f78 100644
--- a/arch/x86/kvm/vmx/vmcs12.c
+++ b/arch/x86/kvm/vmx/vmcs12.c
@@ -115,10 +115,6 @@ const unsigned short vmcs_field_to_offset_table[] = {
FIELD(CR4_GUEST_HOST_MASK, cr4_guest_host_mask),
FIELD(CR0_READ_SHADOW, cr0_read_shadow),
FIELD(CR4_READ_SHADOW, cr4_read_shadow),
- FIELD(CR3_TARGET_VALUE0, cr3_target_value0),
- FIELD(CR3_TARGET_VALUE1, cr3_target_value1),
- FIELD(CR3_TARGET_VALUE2, cr3_target_value2),
- FIELD(CR3_TARGET_VALUE3, cr3_target_value3),
FIELD(EXIT_QUALIFICATION, exit_qualification),
FIELD(GUEST_LINEAR_ADDRESS, guest_linear_address),
FIELD(GUEST_CR0, guest_cr0),
diff --git a/arch/x86/kvm/vmx/vmcs12.h b/arch/x86/kvm/vmx/vmcs12.h
index d0c6df373f67..80232daf00ff 100644
--- a/arch/x86/kvm/vmx/vmcs12.h
+++ b/arch/x86/kvm/vmx/vmcs12.h
@@ -80,10 +80,7 @@ struct __packed vmcs12 {
natural_width cr4_guest_host_mask;
natural_width cr0_read_shadow;
natural_width cr4_read_shadow;
- natural_width cr3_target_value0;
- natural_width cr3_target_value1;
- natural_width cr3_target_value2;
- natural_width cr3_target_value3;
+ natural_width dead_space[4]; /* Last remnants of cr3_target_value[0-3]. */
natural_width exit_qualification;
natural_width guest_linear_address;
natural_width guest_cr0;
@@ -263,10 +260,7 @@ static inline void vmx_check_vmcs12_offsets(void)
CHECK_OFFSET(cr4_guest_host_mask, 352);
CHECK_OFFSET(cr0_read_shadow, 360);
CHECK_OFFSET(cr4_read_shadow, 368);
- CHECK_OFFSET(cr3_target_value0, 376);
- CHECK_OFFSET(cr3_target_value1, 384);
- CHECK_OFFSET(cr3_target_value2, 392);
- CHECK_OFFSET(cr3_target_value3, 400);
+ CHECK_OFFSET(dead_space, 376);
CHECK_OFFSET(exit_qualification, 408);
CHECK_OFFSET(guest_linear_address, 416);
CHECK_OFFSET(guest_cr0, 424);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index aa1b8cf7c915..1ba7722445cc 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5711,7 +5711,6 @@ void dump_vmcs(void)
u32 cpu_based_exec_ctrl, pin_based_exec_ctrl, secondary_exec_control;
unsigned long cr4;
u64 efer;
- int i, n;
if (!dump_invalid_vmcs) {
pr_warn_ratelimited("set kvm_intel.dump_invalid_vmcs=1 to dump internal KVM state.\n");
@@ -5848,14 +5847,6 @@ void dump_vmcs(void)
pr_err("PostedIntrVec = 0x%02x\n", vmcs_read16(POSTED_INTR_NV));
if ((secondary_exec_control & SECONDARY_EXEC_ENABLE_EPT))
pr_err("EPT pointer = 0x%016llx\n", vmcs_read64(EPT_POINTER));
- n = vmcs_read32(CR3_TARGET_COUNT);
- for (i = 0; i + 1 < n; i += 4)
- pr_err("CR3 target%u=%016lx target%u=%016lx\n",
- i, vmcs_readl(CR3_TARGET_VALUE0 + i * 2),
- i + 1, vmcs_readl(CR3_TARGET_VALUE0 + i * 2 + 2));
- if (i < n)
- pr_err("CR3 target%u=%016lx\n",
- i, vmcs_readl(CR3_TARGET_VALUE0 + i * 2));
if (secondary_exec_control & SECONDARY_EXEC_PAUSE_LOOP_EXITING)
pr_err("PLE Gap=%08x Window=%08x\n",
vmcs_read32(PLE_GAP), vmcs_read32(PLE_WINDOW));
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>