Re: [PATCH v2 07/10] KVM: x86: Always populate L1 GPA for KVM_MAP_MEMORY

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Mon, 2024-04-15 at 14:17 -0700, Sean Christopherson wrote:
> > But doesn't the fault handler need the vCPU state?
> 
> Ignoring guest MTRRs, which will hopefully soon be a non-issue, no.  There are
> only six possible roots if TDP is enabled:
> 
>       1. 4-level !SMM !guest_mode
>       2. 4-level  SMM !guest_mode
>       3. 5-level !SMM !guest_mode
>       4. 5-level  SMM !guest_mode
>       5. 4-level !SMM guest_mode
>       6. 5-level !SMM guest_mode
> 
> 4-level vs. 5-level is a guest MAXPHYADDR thing, and swapping the MMU
> eliminates
> the SMM and guest_mode issues.  If there is per-vCPU state that makes its way
> into
> the TDP page tables, then we have problems, because it means that there is
> per-vCPU
> state in per-VM structures that isn't accounted for.
> 
> There are a few edge cases where KVM treads carefully, e.g. if the fault is to
> the vCPU's APIC-access page, but KVM manually handles those to avoid consuming
> per-vCPU state.
> 
> That said, I think this option is effectively 1b, because dropping the SMM vs.
> guest_mode state has the same uAPI problems as forcibly swapping the MMU, it's
> just a different way of doing so.
> 
> The first question to answer is, do we want to return an error or "silently"
> install mappings for !SMM, !guest_mode.  And so this option becomes relevant
> only
> _if_ we want to unconditionally install mappings for the 'base" mode.

Ah, I thought there was some logic around CR0.CD.

> 
> > > - Return error on guest mode or SMM mode:  Without this patch.
> > >   Pros: No additional patch.
> > >   Cons: Difficult to use.
> > 
> > Hmm... For the non-TDX use cases this is just an optimization, right? For
> > TDX
> > there shouldn't be an issue. If so, maybe this last one is not so horrible.
> 
> And the fact there are so variables to control (MAXPHADDR, SMM, and
> guest_mode)
> basically invalidates the argument that returning an error makes the ioctl()
> hard
> to use.  I can imagine it might be hard to squeeze this ioctl() into QEMU's
> existing code, but I don't buy that the ioctl() itself is hard to use.
> 
> Literally the only thing userspace needs to do is set CPUID to implicitly
> select
> between 4-level and 5-level paging.  If userspace wants to pre-map memory
> during
> live migration, or when jump-starting the guest with pre-defined state, simply
> pre-map memory before stuffing guest state.  In and of itself, that doesn't
> seem
> difficult, e.g. at a quick glance, QEMU could add a hook somewhere in
> kvm_vcpu_thread_fn() without too much trouble (though that comes with a huge
> disclaimer that I only know enough about how QEMU manages vCPUs to be
> dangerous).
> 
> I would describe the overall cons for this patch versus returning an error
> differently.  Switching MMU state puts the complexity in the kernel. 
> Returning
> an error punts any complexity to userspace.  Specifically, anything that KVM
> can
> do regarding vCPU state to get the right MMU, userspace can do too.
>  
> Add on that silently doing things that effectively ignore guest state usually
> ends badly, and I don't see a good argument for this patch (or any variant
> thereof).

Great.