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Date: Wed, 5 Jun 2019 18:56:59 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
Radim Krčmář <rkrcmar@...hat.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 2/3] KVM: X86: Provide a capability to disable cstate
msr read intercepts
On Wed, 5 Jun 2019 at 00:53, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 21/05/19 08:06, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Allow guest reads CORE cstate when exposing host CPU power management capabilities
> > to the guest. PKG cstate is restricted to avoid a guest to get the whole package
> > information in multi-tenant scenario.
> >
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Radim Krčmář <rkrcmar@...hat.com>
> > Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> > Cc: Liran Alon <liran.alon@...cle.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> > v1 -> v2:
> > * use a separate bit for KVM_CAP_X86_DISABLE_EXITS
> >
> > Documentation/virtual/kvm/api.txt | 1 +
> > arch/x86/include/asm/kvm_host.h | 1 +
> > arch/x86/kvm/vmx/vmx.c | 6 ++++++
> > arch/x86/kvm/x86.c | 5 ++++-
> > arch/x86/kvm/x86.h | 5 +++++
> > include/uapi/linux/kvm.h | 4 +++-
> > tools/include/uapi/linux/kvm.h | 4 +++-
> > 7 files changed, 23 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index 33cd92d..91fd86f 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -4894,6 +4894,7 @@ Valid bits in args[0] are
> > #define KVM_X86_DISABLE_EXITS_MWAIT (1 << 0)
> > #define KVM_X86_DISABLE_EXITS_HLT (1 << 1)
> > #define KVM_X86_DISABLE_EXITS_PAUSE (1 << 2)
> > +#define KVM_X86_DISABLE_EXITS_CSTATE (1 << 3)
> >
> > Enabling this capability on a VM provides userspace with a way to no
> > longer intercept some instructions for improved latency in some
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index d5457c7..1ce8289 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -882,6 +882,7 @@ struct kvm_arch {
> > bool mwait_in_guest;
> > bool hlt_in_guest;
> > bool pause_in_guest;
> > + bool cstate_in_guest;
> >
> > unsigned long irq_sources_bitmap;
> > s64 kvmclock_offset;
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 0861c71..da24f18 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6637,6 +6637,12 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
> > 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);
>
> I think I have changed my mind on the implementation of this, sorry.
>
> 1) We should emulate these MSRs always, otherwise the guest API changes
> between different values of KVM_CAP_X86_DISABLE_EXITS which is not
> intended. Also, KVM_CAP_X86_DISABLE_EXITS does not prevent live
> migration, so it should be possible to set the MSRs in the host to
> change the delta between the host and guest values.
>
> 2) If both KVM_X86_DISABLE_EXITS_HLT and KVM_X86_DISABLE_EXITS_MWAIT are
> disabled (i.e. exit happens), the MSRs will be purely emulated.
> C3/C6/C7 residency will never increase (it will remain the value that is
> set by the host). When the VM executes an hlt vmexit, it should save
> the current TSC. When it comes back, the C1 residency MSR should be
> increased by the time that has passed.
>
> 3) If KVM_X86_DISABLE_EXITS_HLT is enabled but
> KVM_X86_DISABLE_EXITS_MWAIT is disabled (i.e. mait exits happen),
> C3/C6/C7 residency will also never increase, but the C1 residency value
> should be read using rdmsr from the host, with a delta added from the
> host value.
>
> 4) If KVM_X86_DISABLE_EXITS_HLT and KVM_X86_DISABLE_EXITS_MWAIT are both
> disabled (i.e. mwait exits do not happen), all four residency values
> should be read using rdmsr from the host, with a delta added from the
> host value.
>
> 5) If KVM_X86_DISABLE_EXITS_HLT is disabled and
> KVM_X86_DISABLE_EXITS_MWAIT is enabled, the configuration makes no sense
> so it's okay not to be very optimized. In this case, the residency
> value should be read as in (4), but hlt vmexits will be accounted as in
> (2) so we need to be careful not to double-count the residency during
> hlt. This means doing four rdmsr before the beginning of the hlt vmexit
> and four at the end of the hlt vmexit.
I will have a try, thanks Paolo! :)
Regards,
Wanpeng Li
>
> Therefore the data structure should be something like
>
> struct kvm_residency_msr {
> u64 value;
> bool delta_from_host;
> bool count_with_host;
> }
>
> u64 kvm_residency_read_host(struct kvm_residency_msr *msr)
> {
> u64 unscaled_value = rdmsrl(msr->index);
> // apply TSC scaling...
> return ...
> }
>
> u64 kvm_residency_read(struct kvm_residency_msr *msr)
> {
> return msr->value +
> (msr->delta_from_host ? kvm_residency_read_host(msr) : 0);
> }
>
> void kvm_residency_write(struct kvm_residency_msr *msr,
> u64 value)
> {
> msr->value = value -
> (msr->delta_from_host ? kvm_residency_read_host(msr) : 0);
> }
>
> // count_with_host is true for C1 iff any of KVM_CAP_DISABLE_EXITS_HLT
> // or KVM_CAP_DISABLE_EXITS_MWAIT is set
> // count_with_host is true for C3/C6/C7 iff KVM_CAP_DISABLE_EXITS_MWAIT
> is set
> void kvm_residency_setup(struct kvm_residency_msr *msr, u16 index,
> bool count_with_host)
> {
> /* Preserve value on calls after the first */
> u64 value = msr->index ? kvm_residency_read(msr) : 0;
> msr->delta_from_host = msr->count_with_host = count_with_host;
> msr->index = index;
> kvm_residency_write(msr, value);
> }
>
> // The following functions are called from hlt vmexits.
>
> void kvm_residency_start_hlt(struct kvm_residency_msr *msr)
> {
> if (msr->count_with_host) {
> WARN_ON(msr->delta_from_host);
> msr->value += kvm_residency_read_host(msr);
> msr->delta_from_host = false;
> }
> }
>
> // host_tsc_waited is 0 except for MSR_CORE_C1_RES
> void kvm_residency_end_hlt(struct kvm_residency_msr *msr,
> u64 host_tsc_waited)
> {
> if (msr->count_with_host) {
> WARN_ON(!msr->delta_from_host);
> msr->value -= kvm_residency_read_host(msr);
> msr->delta_from_host = true;
> }
> if (host_tsc_waited) {
> // ... apply TSC scaling to host_tsc_waited ...
> msr->value += ...;
> }
> }
>
> Thanks,
>
> Paolo
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