Re: [PATCH 0/8] KVM: VMX: Implement nested TSC scaling

From: Maxim Levitsky <mlevitsk@redhat.com>
To: "Stamatis, Ilias" <ilstam@amazon.com>,
	"jmattson@google.com" <jmattson@google.com>,
	"ilstam@mailbox.org" <ilstam@mailbox.org>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Woodhouse, David" <dwmw@amazon.co.uk>,
	"vkuznets@redhat.com" <vkuznets@redhat.com>,
	"mtosatti@redhat.com" <mtosatti@redhat.com>,
	"joro@8bytes.org" <joro@8bytes.org>,
	"zamsden@gmail.com" <zamsden@gmail.com>,
	"seanjc@google.com" <seanjc@google.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"wanpengli@tencent.com" <wanpengli@tencent.com>
Subject: Re: [PATCH 0/8] KVM: VMX: Implement nested TSC scaling
Date: Mon, 10 May 2021 16:43:22 +0300	[thread overview]
Message-ID: <132ed17999bd1464c6c43fbe4e7933ce7cc9eeb9.camel@redhat.com> (raw)
In-Reply-To: <fccb8b01aadfb7e53f8711100bc10dc1c98c5cd5.camel@amazon.com>

On Thu, 2021-05-06 at 17:48 +0000, Stamatis, Ilias wrote:
> On Thu, 2021-05-06 at 10:16 -0700, Jim Mattson wrote:
> > On Thu, May 6, 2021 at 3:34 AM <ilstam@mailbox.org> wrote:
> > > From: Ilias Stamatis <ilstam@amazon.com>
> > > 
> > > KVM currently supports hardware-assisted TSC scaling but only for L1
> > > and it
> > > doesn't expose the feature to nested guests. This patch series adds
> > > support for
> > > nested TSC scaling and allows both L1 and L2 to be scaled with
> > > different
> > > scaling factors.
> > > 
> > > When scaling and offsetting is applied, the TSC for the guest is
> > > calculated as:
> > > 
> > > (TSC * multiplier >> 48) + offset
> > > 
> > > With nested scaling the values in VMCS01 and VMCS12 need to be
> > > merged
> > > together and stored in VMCS02.
> > > 
> > > The VMCS02 values are calculated as follows:
> > > 
> > > offset_02 = ((offset_01 * mult_12) >> 48) + offset_12
> > > mult_02 = (mult_01 * mult_12) >> 48
> > > 
> > > The last patch of the series adds a KVM selftest.
> > 
> > Will you be doing the same for SVM? The last time I tried to add a
> > nested virtualization feature for Intel only, Paolo rapped my knuckles
> > with a ruler.
> 
> Yes, I can try do this, if it's not much more complicated, once I get
> access to AMD hardware. 

I have access to AMD hardware with regular TSC scaling,
and nested TSC scaling IMHO won't be hard for me to implement 
so I volunteer for this task! 

Best regards,
	Maxim Levitsky

> 
> But I suppose this series is standalone and could be merged separately?
> By taking a quick look it seems that SVM exposes far less features to
> nested guests than VMX does anyway.
> 
> Ilias