Re: [PATCH v2 3/3] KVM: VMX: Some minor refactor of MSR bitmap

From: Xiaoyao Li
Date: Fri Oct 18 2019 - 21:28:50 EST

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In reply to: Sean Christopherson: "Re: [PATCH v2 3/3] KVM: VMX: Some minor refactor of MSR bitmap"
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On 10/19/2019 1:27 AM, Sean Christopherson wrote:

On Fri, Oct 18, 2019 at 05:37:23PM +0800, Xiaoyao Li wrote:

Move the MSR bitmap capability check from vmx_disable_intercept_for_msr()
and vmx_enable_intercept_for_msr(), so that we can do the check far
early before we really want to touch the bitmap.

Also, we can move the common MSR not-intercept setup to where msr bitmap
is actually used.

Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
Changes in v2:
- Remove the check of cpu_has_vmx_msr_bitmap() from
vmx_{disable,enable}_intercept_for_msr (Krish)
---
arch/x86/kvm/vmx/vmx.c | 65 +++++++++++++++++++++---------------------
1 file changed, 33 insertions(+), 32 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b083316a598d..017689d0144e 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -343,8 +343,8 @@ module_param_cb(vmentry_l1d_flush, &vmentry_l1d_flush_ops, NULL, 0644);
static bool guest_state_valid(struct kvm_vcpu *vcpu);
static u32 vmx_segment_access_rights(struct kvm_segment *var);
-static __always_inline void vmx_disable_intercept_for_msr(unsigned long *msr_bitmap,
- u32 msr, int type);
+static __always_inline void vmx_set_intercept_for_msr(unsigned long *msr_bitmap,
+ u32 msr, int type, bool value);
void vmx_vmexit(void);
@@ -2000,9 +2000,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
* in the merging. We update the vmcs01 here for L1 as well
* since it will end up touching the MSR anyway now.
*/
- vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap,
- MSR_IA32_SPEC_CTRL,
- MSR_TYPE_RW);
+ vmx_set_intercept_for_msr(vmx->vmcs01.msr_bitmap,
+ MSR_IA32_SPEC_CTRL,
+ MSR_TYPE_RW, false);

IMO this is a net negative. The explicit "disable" is significantly more
intuitive than "set" with a %false param, e.g. at a quick glance it would
be easy to think this code is "setting", i.e. "enabling" interception.

How about renaming it to vmx_switch_intercept_for_msr()?
or just add the cpu_has_vmx_msr_bitmap() check outside because the check is removed in vmx_disable_intercept_for_msr()

break;
case MSR_IA32_PRED_CMD:
if (!msr_info->host_initiated &&
@@ -2028,8 +2028,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
* vmcs02.msr_bitmap here since it gets completely overwritten
* in the merging.
*/
- vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_IA32_PRED_CMD,
- MSR_TYPE_W);
+ vmx_set_intercept_for_msr(vmx->vmcs01.msr_bitmap,
+ MSR_IA32_PRED_CMD,
+ MSR_TYPE_W, false);
break;
case MSR_IA32_CR_PAT:
if (!kvm_pat_valid(data))
@@ -3599,9 +3600,6 @@ static __always_inline void vmx_disable_intercept_for_msr(unsigned long *msr_bit
{
int f = sizeof(unsigned long);
- if (!cpu_has_vmx_msr_bitmap())
- return;

As above, I'd rather keep these here. Functionally it changes nothing on
CPUs with an MSR bitmap. For old CPUs, it saves all of two uops in paths
that aren't performance critical.

-
if (static_branch_unlikely(&enable_evmcs))
evmcs_touch_msr_bitmap();
@@ -3637,9 +3635,6 @@ static __always_inline void vmx_enable_intercept_for_msr(unsigned long *msr_bitm
{
int f = sizeof(unsigned long);
- if (!cpu_has_vmx_msr_bitmap())
- return;
-
if (static_branch_unlikely(&enable_evmcs))
evmcs_touch_msr_bitmap();
@@ -3673,6 +3668,9 @@ static __always_inline void vmx_enable_intercept_for_msr(unsigned long *msr_bitm
static __always_inline void vmx_set_intercept_for_msr(unsigned long *msr_bitmap,
u32 msr, int type, bool value)
{
+ if (!cpu_has_vmx_msr_bitmap())
+ return;
+
if (value)
vmx_enable_intercept_for_msr(msr_bitmap, msr, type);
else
@@ -4163,11 +4161,30 @@ static void ept_set_mmio_spte_mask(void)
static void vmx_vmcs_setup(struct vcpu_vmx *vmx)
{
+ unsigned long *msr_bitmap;
+
if (nested)
nested_vmx_vmcs_setup();
- if (cpu_has_vmx_msr_bitmap())
+ if (cpu_has_vmx_msr_bitmap()) {
+ msr_bitmap = vmx->vmcs01.msr_bitmap;
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_TSC, MSR_TYPE_R);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_FS_BASE, MSR_TYPE_RW);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_GS_BASE, MSR_TYPE_RW);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_KERNEL_GS_BASE, MSR_TYPE_RW);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_SYSENTER_CS, MSR_TYPE_RW);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_SYSENTER_ESP, MSR_TYPE_RW);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_SYSENTER_EIP, MSR_TYPE_RW);
+ if (kvm_cstate_in_guest(vmx->vcpu.kvm)) {
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C1_RES, MSR_TYPE_R);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C3_RESIDENCY, MSR_TYPE_R);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C6_RESIDENCY, MSR_TYPE_R);
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C7_RESIDENCY, MSR_TYPE_R);
+ }
+
vmcs_write64(MSR_BITMAP, __pa(vmx->vmcs01.msr_bitmap));
+ }
+ vmx->msr_bitmap_mode = 0;

Zeroing msr_bitmap_mode can be skipped as well.

vmcs_write64(VMCS_LINK_POINTER, -1ull); /* 22.3.1.5 */
@@ -6074,7 +6091,8 @@ void vmx_set_virtual_apic_mode(struct kvm_vcpu *vcpu)
}
secondary_exec_controls_set(vmx, sec_exec_control);
- vmx_update_msr_bitmap(vcpu);
+ if (cpu_has_vmx_msr_bitmap())
+ vmx_update_msr_bitmap(vcpu);
}
static void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu, hpa_t hpa)
@@ -6688,7 +6706,6 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
{
int err;
struct vcpu_vmx *vmx;
- unsigned long *msr_bitmap;
int i, cpu;
BUILD_BUG_ON_MSG(offsetof(struct vcpu_vmx, vcpu) != 0,
@@ -6745,22 +6762,6 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
if (err < 0)
goto free_msrs;
- msr_bitmap = vmx->vmcs01.msr_bitmap;
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_TSC, MSR_TYPE_R);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_FS_BASE, MSR_TYPE_RW);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_GS_BASE, MSR_TYPE_RW);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_KERNEL_GS_BASE, MSR_TYPE_RW);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_SYSENTER_CS, MSR_TYPE_RW);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_SYSENTER_ESP, MSR_TYPE_RW);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_SYSENTER_EIP, MSR_TYPE_RW);
- if (kvm_cstate_in_guest(kvm)) {
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C1_RES, MSR_TYPE_R);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C3_RESIDENCY, MSR_TYPE_R);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C6_RESIDENCY, MSR_TYPE_R);
- vmx_disable_intercept_for_msr(msr_bitmap, MSR_CORE_C7_RESIDENCY, MSR_TYPE_R);
- }
- vmx->msr_bitmap_mode = 0;

Keep this code here to be consistent with the previous change that moved
the guest_msrs intialization *out* of the VMCS specific function. Both
are collateral pages that are not directly part of the VMCS.

I'd be tempted to use a goto to skip the code, the line length is bad
enough as it is, e.g.:

if (!cpu_has_vmx_msr_bitmap())
goto skip_msr_bitmap;

vmx->msr_bitmap_mode = 0;
skip_msr_bitmap:

-
vmx->loaded_vmcs = &vmx->vmcs01;
cpu = get_cpu();
vmx_vcpu_load(&vmx->vcpu, cpu);
--
2.19.1

Next message: Vijay Khemka: "Re: [PATCH v2] ftgmac100: Disable HW checksum generation on AST2500"
Previous message: Joe Perches: "Re: [PATCH] omapfb: reduce stack usage"
In reply to: Sean Christopherson: "Re: [PATCH v2 3/3] KVM: VMX: Some minor refactor of MSR bitmap"
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