Re: [Xen-devel] [PATCH v9 11/15] microcode: unify loading update during CPU resuming and AP wakeup

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Aug 29, 2019 at 03:37:47PM +0800, Chao Gao wrote:
> On Fri, Aug 23, 2019 at 11:09:07AM +0200, Roger Pau Monné wrote:
> >On Fri, Aug 23, 2019 at 12:44:34AM +0800, Chao Gao wrote:
> >> On Thu, Aug 22, 2019 at 04:10:46PM +0200, Roger Pau Monné wrote:
> >> >On Mon, Aug 19, 2019 at 09:25:24AM +0800, Chao Gao wrote:
> >> >> Both are loading the cached patch. Since APs call the unified function,
> >> >> microcode_update_one(), during wakeup, the 'start_update' parameter
> >> >> which originally used to distinguish BSP and APs is redundant. So remove
> >> >> this parameter.
> >> >> 
> >> >> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> >> >> ---
> >> >> Note that here is a functional change: resuming a CPU would call
> >> >> ->end_update() now while previously it wasn't. Not quite sure
> >> >> whether it is correct.
> >> >
> >> >I guess that's required if it called start_update prior to calling
> >> >end_update?
> >> >
> >> >> 
> >> >> Changes in v9:
> >> >>  - return -EOPNOTSUPP rather than 0 if microcode_ops is NULL in
> >> >>    microcode_update_one()
> >> >>  - rebase and fix conflicts.
> >> >> 
> >> >> Changes in v8:
> >> >>  - split out from the previous patch
> >> >> ---
> >> >>  xen/arch/x86/acpi/power.c       |  2 +-
> >> >>  xen/arch/x86/microcode.c        | 90 
> >> >> ++++++++++++++++++-----------------------
> >> >>  xen/arch/x86/smpboot.c          |  5 +--
> >> >>  xen/include/asm-x86/processor.h |  4 +-
> >> >>  4 files changed, 44 insertions(+), 57 deletions(-)
> >> >> 
> >> >> diff --git a/xen/arch/x86/acpi/power.c b/xen/arch/x86/acpi/power.c
> >> >> index 4f21903..24798d5 100644
> >> >> --- a/xen/arch/x86/acpi/power.c
> >> >> +++ b/xen/arch/x86/acpi/power.c
> >> >> @@ -253,7 +253,7 @@ static int enter_state(u32 state)
> >> >>  
> >> >>      console_end_sync();
> >> >>  
> >> >> -    microcode_resume_cpu();
> >> >> +    microcode_update_one();
> >> >>  
> >> >>      if ( !recheck_cpu_features(0) )
> >> >>          panic("Missing previously available feature(s)\n");
> >> >> diff --git a/xen/arch/x86/microcode.c b/xen/arch/x86/microcode.c
> >> >> index a2febc7..bdd9c9f 100644
> >> >> --- a/xen/arch/x86/microcode.c
> >> >> +++ b/xen/arch/x86/microcode.c
> >> >> @@ -203,24 +203,6 @@ static struct microcode_patch *parse_blob(const 
> >> >> char *buf, uint32_t len)
> >> >>      return NULL;
> >> >>  }
> >> >>  
> >> >> -int microcode_resume_cpu(void)
> >> >> -{
> >> >> -    int err;
> >> >> -    struct cpu_signature *sig = &this_cpu(cpu_sig);
> >> >> -
> >> >> -    if ( !microcode_ops )
> >> >> -        return 0;
> >> >> -
> >> >> -    spin_lock(&microcode_mutex);
> >> >> -
> >> >> -    err = microcode_ops->collect_cpu_info(sig);
> >> >> -    if ( likely(!err) )
> >> >> -        err = microcode_ops->apply_microcode(microcode_cache);
> >> >> -    spin_unlock(&microcode_mutex);
> >> >> -
> >> >> -    return err;
> >> >> -}
> >> >> -
> >> >>  void microcode_free_patch(struct microcode_patch *microcode_patch)
> >> >>  {
> >> >>      microcode_ops->free_patch(microcode_patch->mc);
> >> >> @@ -384,11 +366,29 @@ static int __init microcode_init(void)
> >> >>  }
> >> >>  __initcall(microcode_init);
> >> >>  
> >> >> -int __init early_microcode_update_cpu(bool start_update)
> >> >> +/* Load a cached update to current cpu */
> >> >> +int microcode_update_one(void)
> >> >> +{
> >> >> +    int rc;
> >> >> +
> >> >> +    if ( !microcode_ops )
> >> >> +        return -EOPNOTSUPP;
> >> >> +
> >> >> +    rc = microcode_update_cpu(NULL);
> >> >> +
> >> >> +    if ( microcode_ops->end_update )
> >> >> +        microcode_ops->end_update();
> >> >
> >> >Don't you need to call start_update before calling
> >> >microcode_update_cpu?
> >> 
> >> No. On AMD side, osvw_status records the hardware erratum in the system.
> >> As we don't assume all CPUs have the same erratum, each cpu calls
> >> end_update to update osvw_status after ucode loading.
> >> start_update just resets osvw_status to 0. And it is called once prior
> >> to ucode loading on any CPU so that osvw_status can be recomputed.
> >
> >Oh, I think I understand it. start_update must only be called once
> >_before_ the sequence to update the microcode on all CPUs is
> >performed, while end_update needs to be called on _each_ CPU after the
> >update has been completed in order to account for any erratas.
> >
> >The name for those hooks should be improved, I guess renaming
> >end_update to end_update_each or end_update_percpu would be clearer in
> >order to make it clear that start_update is global, while end_update
> >is percpu. Anyway, I don't want to delay this series for a naming nit.
> >
> >I'm still unsure where start_update is called for the resume from
> >suspension case, I don't seem to see any call to start_update neither
> >in enter_state or microcode_update_one, hence I think this is missing?
> 
> No. Actually, no call of start_update for resume case.
> 
> >
> >I would expect you need to clean osvw_status also on resume from
> >suspension, in case microcode loading fails? Or else you will be
> >carrying a stale osvw_status.
> 
> Then we need to send IPI to all other CPUs to recompute osvw_state.

Why would you need to send an IPI? Aren't other CPUs going to update
the microcode, and hence call end_update?

AFAICT you only need to call start_update after returning from
suspension and before any CPU updates it's microcode. Then osvw_status
will be updated by each CPU as the microcode gets loaded?

> But
> I think it is not necessary. If ucode cache isn't changed during the
> CPU's suspension period, there is not stale osvw bit (assuming OSVW on
> the resuming CPU won't change). If the ucode cache is updated (there
> must be a late ucode loading), osvw_status should have been cleaned
> before late ucode loading.

It could be possible that an ucode that previously loaded fine throw
an error, but I agree that's quite unlikely. Anyway the fix seemed
trivial to me, but maybe I'm missing something.

Thanks, Roger.

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