linux-kernel - Re: [PATCH 1/2] X86/KVM: Properly update 'tsc

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Date:   Sat, 14 Apr 2018 06:31:48 +0000
From:   "Raslan, KarimAllah" <karahmed@...zon.de>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
CC:     "jmattson@...gle.com" <jmattson@...gle.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/2] X86/KVM: Properly update 'tsc_offset' to represent
 the running guest

On Fri, 2018-04-13 at 17:35 +0200, Paolo Bonzini wrote:
> On 13/04/2018 14:40, Raslan, KarimAllah wrote:
> > 
> > > 
> > >  
> > >  static void update_ia32_tsc_adjust_msr(struct kvm_vcpu *vcpu, s64 offset)
> > >  {
> > > -	u64 curr_offset = vcpu->arch.tsc_offset;
> > > +	u64 curr_offset = kvm_x86_ops->read_l1_tsc_offset(vcpu);
> > I might be missing something but is this really strictly needed or is
> > it really a bug?
> > 
> > I can see update_ia32_tsc_adjust_msr called from kvm_write_tsc only 
> > which is called from a) vmx_set_msr or b) kvm_arch_vcpu_postcreate.
> > The adjust_msr would only be called if !host_initiated. So only 
> > vmx_set_msr which is coming from an L1 write (or a restore but that
> > would not be !host_initiated). So the only that tsc_adjust is called is
> > !is_guest_mode.
> 
> It can also be called from guest mode if the MSR bitmap says there's no
> L1 vmexit for that MSR; that's what the testcases do.

Apparently I will never wrap my head around this nested stuff :D

> 
> Paolo
> 
> > 
> > > 
> > >  	vcpu->arch.ia32_tsc_adjust_msr += offset - curr_offset;
> 
> 
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