Date: Thu, 7 Mar 2024 11:06:57 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Yan Zhao <yan.y.zhao@...el.com>, "Isaku
 Yamahata" <isaku.yamahata@...el.com>, Michael Roth <michael.roth@....com>, Yu
 Zhang <yu.c.zhang@...ux.intel.com>, Chao Peng <chao.p.peng@...ux.intel.com>,
	Fuad Tabba <tabba@...gle.com>, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above
 slot validity checks



On 6/03/2024 3:02 pm, Sean Christopherson wrote:
> On Wed, Mar 06, 2024, Kai Huang wrote:
>>
>>
>> On 6/03/2024 1:38 pm, Sean Christopherson wrote:
>>> On Wed, Mar 06, 2024, Kai Huang wrote:
>>>>
>>>>
>>>> On 28/02/2024 3:41 pm, Sean Christopherson wrote:
>>>>> Prioritize private vs. shared gfn attribute checks above slot validity
>>>>> checks to ensure a consistent userspace ABI.  E.g. as is, KVM will exit to
>>>>> userspace if there is no memslot, but emulate accesses to the APIC access
>>>>> page even if the attributes mismatch.
>>>>
>>>> IMHO, it would be helpful to explicitly say that, in the later case (emulate
>>>> APIC access page) we still want to report MEMORY_FAULT error first (so that
>>>> userspace can have chance to fixup, IIUC) instead of emulating directly,
>>>> which will unlikely work.
>>>
>>> Hmm, it's not so much that emulating directly won't work, it's that KVM would be
>>> violating its ABI.  Emulating APIC accesses after userspace converted the APIC
>>> gfn to private would still work (I think), but KVM's ABI is that emulated MMIO
>>> is shared-only.
>>
>> But for (at least) TDX guest I recall we _CAN_ allow guest's MMIO to be
>> mapped as private, right?  The guest is supposed to get a #VE anyway?
> 
> Not really.  KVM can't _map_ emulated MMIO as private memory, because S-EPT
> entries can only point at convertible memory.  

Right.  I was talking about the MMIO mapping in the guest, which can be 
private I suppose.

KVM _could_ emulate in response
> to a !PRESENT EPT violation, but KVM is not going to do that.
> 
> https://lore.kernel.org/all/ZcUO5sFEAIH68JIA@google.com
> 

Right agreed KVM shouldn't "emulate" !PRESENT fault.

I am talking about this -- for TDX guest, if I recall correctly, for 
guest's MMIO gfn KVM still needs to get the EPT violation for the 
_first_ access, in which KVM can configure the EPT entry to make sure 
"suppress #VE" bit is cleared so the later accesses can trigger #VE 
directly.

I suppose this is still the way we want to implement?

I am afraid for this case, we will still see the MMIO GFN as private, 
while by default I believe the "guest memory attributes" for that MMIO 
GFN should be shared?  AFAICT, it seems the "guest memory attributes" 
code doesn't check whether a GFN is MMIO or truly RAM.

So I agree making KVM only "emulate" shared MMIO makes sense, and we 
need this patch to cover the APIC access page case.

Anyway:

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

>> Perhaps I am missing something -- I apologize if this has already been
>> discussed.
>>
>>>
>>> FWIW, I doubt there's a legitmate use case for converting the APIC gfn to private,
>>> this is purely to ensure KVM has simple, consistent rules for how private vs.
>>> shared access work.
>>
>> Again I _think_ for TDX APIC gfn can be private?  IIUC virtualizing APIC is
>> done by the TDX module, which injects #VE to guest when emulation is
>> required.
> 
> It's a moot point for TDX, as x2APIC is mandatory.

Right.  My bad to mention.

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