Re: [PATCH v2 for-4.15] x86/msr: introduce an option for HVM relaxed rdmsr behavior

[Jan Beulich <jbeulich@suse.com>]

On 05.03.2021 10:15, Roger Pau Monné wrote:
> On Fri, Mar 05, 2021 at 12:06:19AM +0000, Andrew Cooper wrote:
>> On 04/03/2021 14:47, Roger Pau Monne wrote:
>>> From a release PoV the biggest risk would be breaking some of the
>>> existing MSR functionality. I think that's a necessary risk in order
>>> to offer such fallback option, or else we might discover after the
>>> release that guests that worked on Xen 4.14 don't work anymore in Xen
>>> 4.15.
>>
>> Much as I'd prefer not to have this, I agree with the sentiment that we
>> should have an "emergency undo" which people can use, and carry it for
>> at least a short while.
>>
>> However, to be useful for the purpose of unbreaking VMs, it must change
>> both the read and write behaviour, because both are potential
>> compatibility concerns (without reintroducing the information leak).
> 
> I think I was confused here and assumed the previous behavior would
> check the written value to match the current underlying value before
> injecting a #GP. That's not the case.
> 
> I can expand this patch to include the write side, I just thought
> having the rad side only would be enough to cover for the unhandled
> MSRs accesses.

Both when seeing this patch's title and when ripping the write part
out of my patch I meant to indicate the same - dealing with just
reads may not be enough. Arguably people could be told to first try
with just relaxing rdmsr handling, but ones anxious to get their
VMs back into production use may ignore such an advice and use the
bigger hammer right away.

Jan