linux-kernel - Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for TDX shared bit of GPA

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Date: Thu, 16 May 2024 13:40:41 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>
CC: "Zhao, Yan Y" <yan.y.zhao@...el.com>, "sagis@...gle.com"
	<sagis@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "dmatlack@...gle.com"
	<dmatlack@...gle.com>
Subject: Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for
 TDX shared bit of GPA

On 16/05/2024 1:20 pm, Edgecombe, Rick P wrote:
> On Thu, 2024-05-16 at 13:04 +1200, Huang, Kai wrote:
>>
>> I really don't see difference between ...
>>
>>          is_private_mem(gpa)
>>
>> ... and
>>
>>          is_private_gpa(gpa)
>>
>> If it confuses me, it can confuses other people.
> 
> Again, point taken. I'll try to think of a better name. Please share if you do.
> 
>>
>> The point is there's really no need to distinguish the two.  The GPA is
>> only meaningful when it refers to the memory that it points to.
>>
>> So far I am not convinced we need this helper, because such info we can
>> already get from:
>>
>>     1) fault->is_private;
>>     2) Xarray which records memtype for given GFN.
>>
>> So we should just get rid of it.
> 
> Kai, can you got look through the dev branch a bit more before making the same
> point on every patch?
> 
> kvm_is_private_gpa() is used to set PFERR_PRIVATE_ACCESS, which in turn sets
> fault->is_private. So you are saying we can use these other things that are
> dependent on it. Look at the other callers too.

Well, I think I didn't make myself clear.

I don't object to have this helper.  If it helps, then we can have it.

My objection is the current implementation of it, because it is 
*conceptually* wrong for SEV-SNP.

Btw, I just look at the dev branch.

For the common code, it is used in kvm_tdp_mmu_map() and 
kvm_tdp_mmu_fast_pf_get_last_sptep() to get whether a GPA is private.

As said above, I don't see why we need a helper with the "current 
implementation" (which consults kvm_shared_gfn_mask()) for them.  We can 
just use fault->gfn + fault->is_private for such purpose.

It is also used in the TDX code like TDX variant handle_ept_violation() 
and tdx_vcpu_init_mem_region().  For them to be honest I don't quite 
care whether a helper is used.  We can have a helper if we have multiple 
callers, but this helper should be in TDX code, but not common MMU code.