linux-kernel - Re: [PATCH v2 06/15] KVM: x86/mmu: Support GFN direct mask

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Message-ID: <38b1eb36f1cc0514221251cc8ede35ad173fe77d.camel@intel.com>
Date: Sun, 9 Jun 2024 23:25:56 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "dmatlack@...gle.com"
	<dmatlack@...gle.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Subject: Re: [PATCH v2 06/15] KVM: x86/mmu: Support GFN direct mask

On Sat, 2024-06-08 at 11:08 +0200, Paolo Bonzini wrote:
> > The downside would be wider distribution of the concerns for dealing with
> > multiple aliases for a GFN. Currently, the behavior to have multiple aliases
> > is
> > implemented in core MMU code. While it's fine to pollute tdx.c with TDX
> > specific
> > knowledge of course, removing the handling of this corner from mmu.c might
> > make
> > it less understandable for non-tdx readers who are working in MMU code.
> > Basically, if a concept fits into some non-TDX abstraction like this, having
> > it
> > in core code seems the better default to me.
> 
> I am not sure why it's an MMU concept that "if you offset the shared
> mappings you cannot implement flush_remote_tlbs_range". It seems more
> like, you need to know what you're doing?
> 
> Right now it makes no difference because you don't set the callback;
> but if you ever wanted to implement flush_remote_tlbs_range as an
> optimization you'd have to remove the condition from the "if". So it's
> better not to have it in the first place.

Yea that's true.

> 
> Perhaps add a comment instead, like:
> 
>      if (!kvm_x86_ops.flush_remote_tlbs_range)
>          return -EOPNOTSUPP;
> 
> +    /*
> +     * If applicable, the callback should flush GFNs both with and without
> +     * the direct-mapping bits.
> +     */
>      return static_call(kvm_x86_flush_remote_tlbs_range)(kvm, gfn, nr_pages);

Ok, works for me.