linux-kernel - Re: [PATCH v19 011/130] KVM: Add new members to struct kvm_gfn

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Date: Fri, 26 Apr 2024 15:57:56 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Yuan, Hang" <hang.yuan@...el.com>, "Huang,
 Kai" <kai.huang@...el.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "sagis@...gle.com" <sagis@...gle.com>, "Chen,
 Bo2" <chen.bo@...el.com>, "isaku.yamahata@...ux.intel.com"
	<isaku.yamahata@...ux.intel.com>, "tabba@...gle.com" <tabba@...gle.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>
Subject: Re: [PATCH v19 011/130] KVM: Add new members to struct kvm_gfn_range
 to operate on

On Fri, 2024-04-26 at 08:28 -0700, Sean Christopherson wrote:
> Hmm, yeah, I buy that argument.  We could even harded further by poisoning '0'
> to force KVM to explicitly.  Aha!  And maybe use a bitmap?
> 
>         enum {
>                 BUGGY_KVM_INVALIDATION          = 0,
>                 PROCESS_SHARED                  = BIT(0),
>                 PROCESS_PRIVATE                 = BIT(1),
>                 PROCESS_PRIVATE_AND_SHARED      = PROCESS_SHARED |
> PROCESS_PRIVATE,
>         };

Seems like it would work for all who have been concerned. The previous objection
to the enum (can't find the mail) was for requiring logic like:

   if (zap == PROCESS_PRIVATE_AND_SHARED || zap == PROCESS_PRIVATE)
   	do_private_zap_stuff();
   
   
We are trying to tie things up internally so we can jointly have something to
stare at again, as the patches are diverging. But will make this adjustment.


> 
> > > https://lore.kernel.org/all/ZUO1Giju0GkUdF0o@google.com/
> > 
> > Currently in our internal branch we switched to:
> > exclude_private
> > exclude_shared
> > 
> > It came together bettter in the code that uses it.
> 
> If the choice is between an enum and exclude_*, I would strongly prefer the
> enum.
> Using exclude_* results in inverted polarity for the code that triggers
> invalidations.

Right, the awkwardness lands in that code.

The processing code looks nice though:
https://lore.kernel.org/kvm/5210e6e6e2eb73b04cb7039084015612479ae2fe.camel@intel.com/

> 
> > But I started to wonder if we actually really need exclude_shared. For TDX
> > zapping private memory has to be done with more care, because it cannot be
> > re-
> > populated without guest coordination. But for shared memory if we are
> > zapping a
> > range that includes both private and shared memory, I don't think it should
> > hurt
> > to zap the shared memory.
> 
> Hell no, I am not risking taking on more baggage in KVM where userspace or
> some
> other subsystem comes to rely on KVM spuriously zapping SPTEs in response to
> an
> unrelated userspace action. 

Hmm, I see the point. Thanks. This was just being left for later discussion
anyway.