Re: [PATCH v8 2/6] KVM: x86: Virtualize CR4.LAM_SUP

[Binbin Wu]

On 5/12/2023 6:49 PM, Huang, Kai wrote:
> > > >    
> > > LAM only applies to 64-bit linear address, which means LAM can only be enabled
> > > when CPU is in 64-bit mode with either 4-level or 5-level paging enabled.
> > >
> > > What's the hardware behaviour if we set CR4.LAM_SUP when CPU isn't in 64-bit
> > > mode?  And how does VMENTRY check GUEST_CR4.LAM_SUP and 64-bit mode?
> > >
> > > Looks they are not clear in the spec you pasted in the cover letter:
> > >
> > > https://cdrdv2.intel.com/v1/dl/getContent/671368
> > >
> > > Or I am missing something?
> > Yes, it is not clearly described in LAM spec.
> > Had some internal discussions and also did some tests in host,
> > if the processor supports LAM, CR4.LAM_SUP is allowed to be set even
> > when cpu isn't in 64bit mode.
> >
> > There was a statement in commit message of the last version, but I
> > missed it in this version. I'll add it back.
> > "CR4.LAM_SUP is allowed to be set even not in 64-bit mode, but it will not
> > take effect since LAM only applies to 64-bit linear address."
> Yeah this does help.  Please add it back to the changelog.
>
> > Also, I will try to ask Intel guys if it's possible to update the document.
> Thanks.


Per the internal discussion, there is no need to explicitly callout 
CR4[28] can be set out side of 64-bit mode in SDM/LAM spec for the 
following reasons:

According to SDM Vol.2 Move to/from Control Registers:
- "On a 64-bit capable processor, an execution of MOV to CR outside of 
64-bit mode zeros the upper 32 bits of the control register."
  It doesn't mention of clearing any of the lower bits.

- "Some of the bits in CR0, CR3, and CR4 are reserved and must be 
written with zeros. ...  Attempting to set any reserved bits in CR4 
results in #GP(0)"
  CR4[28] is not reserved on processors that support LAM, and SDM / LAM 
spec doesn't explicitly say the bit cannot be set under some specific 
condition.

So just like the reset of CR4[31:0], CR4[28] can be set by any 32-bit 
load of CR4 when LAM is supported.
For example, CR4[17] is used only with 64-bit paging, but it can be set 
by a 32-bit load of CR4 when 32-bit paging or PAE paging is in use.
Similarly, user-interrupt delivery is enabled by setting CR4[25]. It can 
be set in any mode, even though user-interrupt delivery can occur only 
in 64-bit mode.