Re: [PATCH 4/4] kvm: x86: export TSC offset to user-space

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 2 Sep 2016 19:00:41 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 31/08/2016 19:05, Luiz Capitulino wrote:
> >   vcpu0: 18446742405270834952
> >   vcpu1: 18446742405270834952
> >   vcpu2: 18446742405270834952
> >   vcpu3: 18446742405270834952
> > 
> >  - We'll probably need to export the TSC multiplier too.
> >    However, I've been using only the TSC offset for now.
> >    So, let's get this merged first and do the TSC multiplier
> >    as a second step  
> 
> You'll need to export the number of fractional bits in the multiplier,
> too.  It's going to be a very simple patch, so please do everything now.

I didn't want to expose the multiplier before testing our tracing
procedure with it. So far we've been only using the TSC offset (and
it works great). I don't even know if I have a machine around to
test it, so it could take a bit.

> arch/x86/kvm/x86.c is huge; please create a new file arch/x86/kvm/debugfs.c.

Will do.

> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  1 +
> >  arch/x86/kvm/svm.c              |  1 +
> >  arch/x86/kvm/vmx.c              |  8 ++++++++
> >  arch/x86/kvm/x86.c              | 30 ++++++++++++++++++++++++++++++
> >  4 files changed, 40 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 33ae3a4..5714bbd 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -952,6 +952,7 @@ struct kvm_x86_ops {
> >  	bool (*has_wbinvd_exit)(void);
> >  
> >  	u64 (*read_tsc_offset)(struct kvm_vcpu *vcpu);
> > +	u64 (*read_cached_tsc_offset)(struct kvm_vcpu *vcpu);
> >  	void (*write_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);
> >  
> >  	u64 (*read_l1_tsc)(struct kvm_vcpu *vcpu, u64 host_tsc);
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index af523d8..c851477 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -5065,6 +5065,7 @@ static struct kvm_x86_ops svm_x86_ops = {
> >  	.has_wbinvd_exit = svm_has_wbinvd_exit,
> >  
> >  	.read_tsc_offset = svm_read_tsc_offset,
> > +	.read_cached_tsc_offset = svm_read_tsc_offset,
> >  	.write_tsc_offset = svm_write_tsc_offset,
> >  	.adjust_tsc_offset_guest = svm_adjust_tsc_offset_guest,
> >  	.read_l1_tsc = svm_read_l1_tsc,
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index 5cede40..82dfe42 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -616,6 +616,7 @@ struct vcpu_vmx {
> >  	u64 hv_deadline_tsc;
> >  
> >  	u64 current_tsc_ratio;
> > +	u64 cached_tsc_offset;
> >  
> >  	bool guest_pkru_valid;
> >  	u32 guest_pkru;
> > @@ -2608,6 +2609,11 @@ static u64 vmx_read_tsc_offset(struct kvm_vcpu *vcpu)
> >  	return vmcs_read64(TSC_OFFSET);
> >  }
> >  
> > +static u64 vmx_read_cached_tsc_offset(struct kvm_vcpu *vcpu)
> > +{
> > +	return to_vmx(vcpu)->cached_tsc_offset;
> > +}
> > +
> >  /*
> >   * writes 'offset' into guest's timestamp counter offset register
> >   */
> > @@ -2632,6 +2638,7 @@ static void vmx_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> >  					   vmcs_read64(TSC_OFFSET), offset);
> >  		vmcs_write64(TSC_OFFSET, offset);
> >  	}
> > +	to_vmx(vcpu)->cached_tsc_offset = offset;
> >  }
> >  
> >  static void vmx_adjust_tsc_offset_guest(struct kvm_vcpu *vcpu, s64 adjustment)
> > @@ -11275,6 +11282,7 @@ static struct kvm_x86_ops vmx_x86_ops = {
> >  	.has_wbinvd_exit = cpu_has_vmx_wbinvd_exit,
> >  
> >  	.read_tsc_offset = vmx_read_tsc_offset,
> > +	.read_cached_tsc_offset = vmx_read_cached_tsc_offset,
> >  	.write_tsc_offset = vmx_write_tsc_offset,
> >  	.adjust_tsc_offset_guest = vmx_adjust_tsc_offset_guest,
> >  	.read_l1_tsc = vmx_read_l1_tsc,  
> 
> You need to handle SVM as well.  So you might as well simplify the code:

SVM is handled:

	+	.read_cached_tsc_offset = svm_read_tsc_offset,

> - add a kvm_vcpu_write_tsc_offset wrapper for kvm_x86_ops->write_tsc_offset
> 
> - add a tsc_offset field in struct kvm_vcpu_arch
> 
> - replace kvm_x86_ops->read_tsc_offset with accesses to the new field

Given that SVM is handled, you still want me to do this?

> Then in a fifth patch export the TSC offset (and multiplier ;)) to
> userspace.
> 
> I'm not very happy about having a single file for all TSC offsets.
> Creating subdirectories under the PID-FD per-VM directory would be nicer
> in the long run.

I think Steven would also prefer that, but some people raised the
concern at KVM Forum that creating per vcpu dirs in debugfs may
consume considerable memory for a system running several dozen
if not hundreds of VMs. This concern seems valid to me, but I
can do either way.