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>
