Re: [PATCH v9 2/6] KVM: x86: Virtualize CR4.LAM_SUP

[Huang, Kai]

On Wed, 2023-06-07 at 12:55 +0800, Binbin Wu wrote:
> 
> On 6/7/2023 11:40 AM, Huang, Kai wrote:
> > On Tue, 2023-06-06 at 17:18 +0800, Binbin Wu wrote:
> > > Move CR4.LAM_SUP out of CR4_RESERVED_BITS and its reservation depends on vcpu
> > > supporting LAM feature or not. Leave the bit intercepted to avoid vmread every
> > > time when KVM fetches its value, with the expectation that guest won't toggle
> > > the bit frequently.
> > KVM only needs to do vmread once to cache guest's CR4, and presumable vmread is
> > a lot cheaper than a VMEXIT.  So I don't see the value of intercepting it if
> > there's no need to do.
> Here is the discussion about the general rule of interception of CR4 bit.
> Sean mentioned:  "As a base
> rule, KVM intercepts CR4 bits unless there's a reason not to, e.g. if 
> the CR4 bit
> in question is written frequently by real guests and/or never consumed 
> by KVM."
> https://lore.kernel.org/all/Y7xA53sLxCwzfvgD@xxxxxxxxxx/
> 
> And CR4.LAM_SUP value will be used to determin the LAM mode when apply 
> LAM masking in instruction emulations / VMExit handlers,
> and if the bit is passed-through, it will be a vmread in these pathes.

Yeah agreed.

> 
> > 
> > But presumably I think we cannot allow guest to own this bit because KVM wants
> > to return a valid CR4 if LAM isn't exposed to guest?  Otherwise guest can still
> > set this bit even LAM isn't exposed to guest.
> > 
> > Am I missing something?
> Right, this is also a reason why the CR4.LAM_SUP bit should be intercepted.
> Will update the justification.
> I suppose this reason is enough for justification, will remove the 
> performance part in changelog.

Anyway,

Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>