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SubjectRe: [PATCH] KVM: vmx: shadow more fields that are read/written on every vmexits
On Wed, Dec 13, 2017 at 01:13:56PM +0100, Paolo Bonzini wrote:
> Compared to when VMCS shadowing was added to KVM, we are reading/writing
> a few more fields: the PML index, the interrupt status and the preemption
> timer value. The first two are because we are exposing more features
> to nested guests, the preemption timer is simply because we have grown
> a new optimization. Adding them to the shadow VMCS field lists reduces
> the cost of a vmexit by about 1000 clock cycles for each field that exists
> on bare metal.
>
> On the other hand, the guest BNDCFGS and TSC offset are not written on
> fast paths, so remove them.

Which guest types? Linux? What about Windows Hyper-V /VMWare and such?

Does it hurt to have them retained?

>
> Suggested-by: Jim Mattson <jmattson@google.com>
> Cc: Jim Mattson <jmattson@google.com>
> Cc: Wanpeng Li <wanpeng.li@hotmail.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx.c | 32 +++++++++++++++++++++++---------
> 1 file changed, 23 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index d2a35a326e47..9fadba5338fd 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -754,11 +754,12 @@ static struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
> GUEST_CS_LIMIT,
> GUEST_CS_BASE,
> GUEST_ES_BASE,
> - GUEST_BNDCFGS,
> + GUEST_PML_INDEX,
> + GUEST_INTR_STATUS,
> + VMX_PREEMPTION_TIMER_VALUE,
> CR0_GUEST_HOST_MASK,
> CR0_READ_SHADOW,
> CR4_READ_SHADOW,
> - TSC_OFFSET,
> EXCEPTION_BITMAP,
> CPU_BASED_VM_EXEC_CONTROL,
> VM_ENTRY_EXCEPTION_ERROR_CODE,
> @@ -4070,9 +4071,22 @@ static void init_vmcs_shadow_fields(void)
> /* No checks for read only fields yet */
>
> for (i = j = 0; i < max_shadow_read_write_fields; i++) {
> + /*
> + * PML and the preemption timer can be emulated, but the
> + * processor cannot vmwrite to fields that don't exist
> + * on bare metal.
> + */
> switch (shadow_read_write_fields[i]) {
> - case GUEST_BNDCFGS:
> - if (!kvm_mpx_supported())
> + case GUEST_PML_INDEX:
> + if (!cpu_has_vmx_pml())
> + continue;
> + break;
> + case VMX_PREEMPTION_TIMER_VALUE:
> + if (!cpu_has_vmx_preemption_timer())
> + continue;
> + break;
> + case GUEST_INTR_STATUS:
> + if (!cpu_has_vmx_apicv())
> continue;
> break;
> default:
> @@ -7068,11 +7082,6 @@ static __init int hardware_setup(void)
> !(cpu_has_vmx_invvpid_single() || cpu_has_vmx_invvpid_global()))
> enable_vpid = 0;
>
> - if (!cpu_has_vmx_shadow_vmcs())
> - enable_shadow_vmcs = 0;
> - if (enable_shadow_vmcs)
> - init_vmcs_shadow_fields();
> -
> if (!cpu_has_vmx_ept() ||
> !cpu_has_vmx_ept_4levels() ||
> !cpu_has_vmx_ept_mt_wb() ||
> @@ -7192,6 +7201,11 @@ static __init int hardware_setup(void)
> kvm_x86_ops->cancel_hv_timer = NULL;
> }
>
> + if (!cpu_has_vmx_shadow_vmcs())
> + enable_shadow_vmcs = 0;
> + if (enable_shadow_vmcs)
> + init_vmcs_shadow_fields();
> +
> kvm_set_posted_intr_wakeup_handler(wakeup_handler);
>
> kvm_mce_cap_supported |= MCG_LMCE_P;
> --
> 1.8.3.1
>

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