Re: [PATCH v2] x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM

[Brian Gerst]

On Fri, Aug 21, 2020 at 10:22 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Fri, Aug 21, 2020 at 06:52:29AM -0400, Paolo Bonzini wrote:
> > From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> >
> > Don't use RDPID in the paranoid entry flow, as it can consume a KVM
> > guest's MSR_TSC_AUX value if an NMI arrives during KVM's run loop.
> >
> > In general, the kernel does not need TSC_AUX because it can just use
> > __this_cpu_read(cpu_number) to read the current processor id.  It can
> > also just block preemption and thread migration at its will, therefore
> > it has no need for the atomic rdtsc+vgetcpu provided by RDTSCP.  For this
> > reason, as a performance optimization, KVM loads the guest's TSC_AUX when
> > a CPU first enters its run loop.  On AMD's SVM, it doesn't restore the
> > host's value until the CPU exits the run loop; VMX is even more aggressive
> > and defers restoring the host's value until the CPU returns to userspace.
> >
> > This optimization obviously relies on the kernel not consuming TSC_AUX,
> > which falls apart if an NMI arrives during the run loop and uses RDPID.
> > Removing it would be painful, as both SVM and VMX would need to context
> > switch the MSR on every VM-Enter (for a cost of 2x WRMSR), whereas using
> > LSL instead RDPID is a minor blip.
> >
> > Both SAVE_AND_SET_GSBASE and GET_PERCPU_BASE are only used in paranoid entry,
> > therefore the patch can just remove the RDPID alternative.
> >
> > Fixes: eaad981291ee3 ("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> > Cc: Chang Seok Bae <chang.seok.bae@xxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Sasha Levin <sashal@xxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: kvm@xxxxxxxxxxxxxxx
> > Reported-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > Debugged-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> >  arch/x86/entry/calling.h | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> > index 98e4d8886f11..ae9b0d4615b3 100644
> > --- a/arch/x86/entry/calling.h
> > +++ b/arch/x86/entry/calling.h
> > @@ -374,12 +374,14 @@ For 32-bit we have the following conventions - kernel is built with
> >   * Fetch the per-CPU GSBASE value for this processor and put it in @reg.
> >   * We normally use %gs for accessing per-CPU data, but we are setting up
> >   * %gs here and obviously can not use %gs itself to access per-CPU data.
> > + *
> > + * Do not use RDPID, because KVM loads guest's TSC_AUX on vm-entry and
> > + * may not restore the host's value until the CPU returns to userspace.
> > + * Thus the kernel would consume a guest's TSC_AUX if an NMI arrives
> > + * while running KVM's run loop.
> >   */
> >  .macro GET_PERCPU_BASE reg:req
> > -     ALTERNATIVE \
> > -             "LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
> > -             "RDPID  \reg", \
>
> This was the only user of the RDPID macro, I assume we want to yank that out
> as well?

No.  That one should be kept until the minimum binutils version is
raised to one that supports the RDPID opcode.

--
Brian Gerst