Re: [Xen-devel] [PATCH v2 07/15] x86: implement set value flow for MBA

[Roger Pau Monn� <roger.pau@xxxxxxxxxx>]

On Thu, Aug 31, 2017 at 05:13:08PM +0800, Yi Sun wrote:
> On 17-08-31 09:30:47, Roger Pau Monn� wrote:
> > On Thu, Aug 31, 2017 at 10:20:10AM +0800, Yi Sun wrote:
> > > On 17-08-30 09:31:04, Roger Pau Monn� wrote:
> > > > On Thu, Aug 24, 2017 at 09:14:41AM +0800, Yi Sun wrote:
> > > Even user inputs a value that does not fulfill HW requirement, HW can 
> > > handle it.
> > > So, we do not need return error to user. Otherwise, user needs to know 
> > > details
> > > of MBA.
> > > 
> > > But the issue here is how we get the actual value and show it to user. 
> > > There are
> > > two ways to do that:
> > > 1. When setting value, check and change it to valid one and save it to 
> > > our cache.
> > 
> > Instead of performing those checks manually, why not simply write the
> > value and read it back to see the one the hardware has actually
> > chosen (and cache it)?
> > 
> So, I guess you prefer optioin 2, right? If so, do we need check if input 
> value
> is valid? I see above comment that you prefer to return an error and leave the
> decision to user.

Yes, I think so. Is reading the MSR in any performance critical path?
Or it's just used by the user-space tools?

> > Is there any risk in writing an invalid value?
> 
> If input value is no more than the thrtl_max got through CPUID, it is fine 
> that
> HW can handle it.

OK, so I would just check for that then.

Roger.

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