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

[Binbin Wu]

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.

> 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.

Thanks.
> If not, your justification of intercepting this bit isn't correct and needs
> update.