Re: [PATCH 1/3] KVM: vmx: speed up MSR bitmap merge

From: Jim Mattson
Date: Wed Dec 20 2017 - 13:02:46 EST

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>

On Wed, Dec 20, 2017 at 4:05 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> The bulk of the MSR bitmap is either immutable, or can be copied from
> the L1 bitmap. By initializing it at VMXON time, and copying the mutable
> parts one long at a time on vmentry (rather than one bit), about 4000
> clock cycles (30%) can be saved on a nested VMLAUNCH/VMRESUME.
>
> The resulting for loop only has four iterations, so it is cheap enough
> to reinitialize the MSR write bitmaps on every iteration, and it makes
> the code simpler.
>
> Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> v1->v2: do not WARN in nested_vmx_merge_msr_bitmap [David]
> rename function to nested_vmx_prepare_msr_bitmap,
> it's used even if there's no L1 bitmap [Paolo]
>
> arch/x86/kvm/vmx.c | 78 +++++++++++++++++++++++++++++-------------------------
> 1 file changed, 42 insertions(+), 36 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 669f5f74857d..9f9c3194440f 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5183,11 +5183,6 @@ static void nested_vmx_disable_intercept_for_msr(unsigned long *msr_bitmap_l1,
> {
> int f = sizeof(unsigned long);
>
> - if (!cpu_has_vmx_msr_bitmap()) {
> - WARN_ON(1);
> - return;
> - }
> -
> /*
> * See Intel PRM Vol. 3, 20.6.9 (MSR-Bitmap Address). Early manuals
> * have the write-low and read-high bitmap offsets the wrong way round.
> @@ -7459,6 +7454,7 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu)
> (unsigned long *)__get_free_page(GFP_KERNEL);
> if (!vmx->nested.msr_bitmap)
> goto out_msr_bitmap;
> + memset(vmx->nested.msr_bitmap, 0xff, PAGE_SIZE);
> }
>
> vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
> @@ -10151,8 +10147,8 @@ static void vmx_inject_page_fault_nested(struct kvm_vcpu *vcpu,
> }
> }
>
> -static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
> - struct vmcs12 *vmcs12);
> +static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
> + struct vmcs12 *vmcs12);
>
> static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu,
> struct vmcs12 *vmcs12)
> @@ -10241,11 +10237,7 @@ static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu,
> (unsigned long)(vmcs12->posted_intr_desc_addr &
> (PAGE_SIZE - 1)));
> }
> - if (cpu_has_vmx_msr_bitmap() &&
> - nested_cpu_has(vmcs12, CPU_BASED_USE_MSR_BITMAPS) &&
> - nested_vmx_merge_msr_bitmap(vcpu, vmcs12))
> - ;
> - else
> + if (!nested_vmx_prepare_msr_bitmap(vcpu, vmcs12))
> vmcs_clear_bits(CPU_BASED_VM_EXEC_CONTROL,
> CPU_BASED_USE_MSR_BITMAPS);
> }
> @@ -10313,14 +10305,19 @@ static int nested_vmx_check_tpr_shadow_controls(struct kvm_vcpu *vcpu,
> * Merge L0's and L1's MSR bitmap, return false to indicate that
> * we do not use the hardware.
> */
> -static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
> - struct vmcs12 *vmcs12)
> +static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
> + struct vmcs12 *vmcs12)
> {
> int msr;
> struct page *page;
> unsigned long *msr_bitmap_l1;
> unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.msr_bitmap;
>
> + /* Nothing to do if the MSR bitmap is not in use. */
> + if (!cpu_has_vmx_msr_bitmap() ||
> + !nested_cpu_has(vmcs12, CPU_BASED_USE_MSR_BITMAPS))
> + return false;
> +
> /* This shortcut is ok because we support only x2APIC MSRs so far. */
> if (!nested_cpu_has_virt_x2apic_mode(vmcs12))
> return false;
> @@ -10328,32 +10325,41 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
> page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->msr_bitmap);
> if (is_error_page(page))
> return false;
> - msr_bitmap_l1 = (unsigned long *)kmap(page);
>
> - memset(msr_bitmap_l0, 0xff, PAGE_SIZE);
> + msr_bitmap_l1 = (unsigned long *)kmap(page);
> + if (nested_cpu_has_apic_reg_virt(vmcs12)) {
> + /*
> + * L0 need not intercept reads for MSRs between 0x800 and 0x8ff, it
> + * just lets the processor take the value from the virtual-APIC page;
> + * take those 256 bits directly from the L1 bitmap.
> + */
> + for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
> + unsigned word = msr / BITS_PER_LONG;
> + msr_bitmap_l0[word] = msr_bitmap_l1[word];
> + msr_bitmap_l0[word + (0x800 / sizeof(long))] = ~0;
> + }
> + } else {
> + for (msr = 0x800; msr <= 0x8ff; msr += BITS_PER_LONG) {
> + unsigned word = msr / BITS_PER_LONG;
> + msr_bitmap_l0[word] = ~0;
> + msr_bitmap_l0[word + (0x800 / sizeof(long))] = ~0;
> + }
> + }
>
> - if (nested_cpu_has_virt_x2apic_mode(vmcs12)) {
> - if (nested_cpu_has_apic_reg_virt(vmcs12))
> - for (msr = 0x800; msr <= 0x8ff; msr++)
> - nested_vmx_disable_intercept_for_msr(
> - msr_bitmap_l1, msr_bitmap_l0,
> - msr, MSR_TYPE_R);
> + nested_vmx_disable_intercept_for_msr(
> + msr_bitmap_l1, msr_bitmap_l0,
> + APIC_BASE_MSR + (APIC_TASKPRI >> 4),
> + MSR_TYPE_W);
>
> + if (nested_cpu_has_vid(vmcs12)) {
> nested_vmx_disable_intercept_for_msr(
> - msr_bitmap_l1, msr_bitmap_l0,
> - APIC_BASE_MSR + (APIC_TASKPRI >> 4),
> - MSR_TYPE_R | MSR_TYPE_W);
> -
> - if (nested_cpu_has_vid(vmcs12)) {
> - nested_vmx_disable_intercept_for_msr(
> - msr_bitmap_l1, msr_bitmap_l0,
> - APIC_BASE_MSR + (APIC_EOI >> 4),
> - MSR_TYPE_W);
> - nested_vmx_disable_intercept_for_msr(
> - msr_bitmap_l1, msr_bitmap_l0,
> - APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
> - MSR_TYPE_W);
> - }
> + msr_bitmap_l1, msr_bitmap_l0,
> + APIC_BASE_MSR + (APIC_EOI >> 4),
> + MSR_TYPE_W);
> + nested_vmx_disable_intercept_for_msr(
> + msr_bitmap_l1, msr_bitmap_l0,
> + APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
> + MSR_TYPE_W);
> }
> kunmap(page);
> kvm_release_page_clean(page);
> --
> 1.8.3.1
>
>