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From: "Raslan, KarimAllah" <karahmed@amazon.de>
To: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>
Cc: "jmattson@google.com" <jmattson@google.com>,
	"rkrcmar@redhat.com" <rkrcmar@redhat.com>
Subject: Re: [PATCH 1/2] X86/KVM: Properly restore 'tsc_offset' when running an L2 guest
Date: Thu, 12 Apr 2018 20:24:32 +0000	[thread overview]
Message-ID: <1523564672.32594.35.camel@amazon.de> (raw)
In-Reply-To: <ca0e9ac3-1f7b-cefb-a0fe-cc8e6f938c6c@redhat.com>

On Thu, 2018-04-12 at 22:21 +0200, Paolo Bonzini wrote:
> On 12/04/2018 19:21, Raslan, KarimAllah wrote:
> > 
> > Now looking further at the code, it seems that everywhere in the code
> > tsc_offset is treated as the L01 TSC_OFFSET.
> > 
> > Like here:
> > 
> >         if (vmcs12->cpu_based_vm_exec_control &
> > CPU_BASED_USE_TSC_OFFSETING)
> >                 vmcs_write64(TSC_OFFSET,
> >                         vcpu->arch.tsc_offset + vmcs12->tsc_offset);
> > 
> > and here:
> > 
> >         vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> > 
> > and here:
> > 
> > u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
> > {
> >         return vcpu->arch.tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
> > }
> > EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
> > 
> > ... would not it be simpler and more inline with the current code to
> > just do what I did above + remove the "+ l1_tsc_offset" + probably
> > document tsc_offset ?
> 
> Problem is, I don't think it's correct. :)  A good start would be to try
> disabling MSR_IA32_TSC interception in KVM, prepare a kvm-unit-tests
> test that reads the MSR, and see if you get the host or guest TSC...

I actually just submitted a patch with your original suggestion
(I hope)  because I realized that adjust tsc was still using the wrong
tsc_offset anyway :)

> 
> Paolo
> 
Amazon Development Center Germany GmbH
Berlin - Dresden - Aachen
main office: Krausenstr. 38, 10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B

      reply	other threads:[~2018-04-12 20:24 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-12 15:12 KarimAllah Ahmed
2018-04-12 15:12 ` [PATCH 2/2] kvm: nVMX: Introduce KVM_CAP_STATE KarimAllah Ahmed
2018-04-12 16:39   ` Paolo Bonzini
2018-04-14 15:56   ` Raslan, KarimAllah
2018-04-14 22:31     ` Raslan, KarimAllah
2018-04-16 16:22   ` Jim Mattson
2018-04-16 17:15     ` Raslan, KarimAllah
2018-04-26 22:28       ` Jim Mattson
2018-04-27 10:03         ` Paolo Bonzini
2018-04-27 15:19           ` Jim Mattson
2018-04-28  0:42             ` Paolo Bonzini
2018-04-12 16:35 ` [PATCH 1/2] X86/KVM: Properly restore 'tsc_offset' when running an L2 guest Paolo Bonzini
2018-04-12 17:04   ` Raslan, KarimAllah
2018-04-12 17:21     ` Raslan, KarimAllah
2018-04-12 20:21       ` Paolo Bonzini
2018-04-12 20:24         ` Raslan, KarimAllah [this message]

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