linux-kernel - Re: [PATCH v5 4/4] KVM: arm64: Initialize the ptdump parser with stage-2 attributes

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Date: Wed, 14 Feb 2024 16:29:45 +0000
From: Sebastian Ene <sebastianene@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: catalin.marinas@....com, gshan@...hat.com, james.morse@....com,
	mark.rutland@....com, maz@...nel.org, rananta@...gle.com,
	ricarkol@...gle.com, ryan.roberts@....com, shahuang@...hat.com,
	suzuki.poulose@....com, will@...nel.org, yuzenghui@...wei.com,
	kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	vdonnefort@...gle.com
Subject: Re: [PATCH v5 4/4] KVM: arm64: Initialize the ptdump parser with
 stage-2 attributes

On Tue, Feb 13, 2024 at 05:10:30PM +0000, Oliver Upton wrote:
> On Tue, Feb 13, 2024 at 04:59:01PM +0000, Sebastian Ene wrote:
> > On Tue, Feb 13, 2024 at 12:42:42AM +0000, Oliver Upton wrote:
> > > On Wed, Feb 07, 2024 at 02:48:33PM +0000, Sebastian Ene wrote:
> 
> [...]
> 
> > > > +
> > > > +	snprintf(marker_msg, MARKER_MSG_LEN, "IPA bits %2u start lvl %1d",
> > > > +		 pgtable->ia_bits, pgtable->start_level);
> > > > +
> > > > +	ipa_addr_marker[0].name = marker_msg;
> > > 
> > > Is the dynamic name worth the added complexity? I see nothing wrong with
> > > exposing additional debugfs files for simple attributes like the IPA
> > > range and page table levels.
> > > 
> > > I know it isn't *that* much, just looking for every opportunity to
> > > simplify further.
> > > 
> > 
> > We can keep them separate, I have no strong opinion about this. I think
> > this was Vincent's, original suggestion to have them so I will check with
> > him as well.
> 
> Well, if we get to the place where there's a single struct containing
> all of the required data upfront then this becomes less of an issue.
> This is useful information still, so let's see if we can go about it the
> other way.
> 

Allright, I will create separate entries for getting the ipa_space and the start
level which will be exposed in the vm debugfs dir.

> > > > +	ret = kvm_ptdump_parser_init(&parser_state, mmu->pgt, m);
> > > > +	if (ret)
> > > > +		return ret;
> > > > +
> > > 
> > > Can this be done at open(), or am I missing something?
> > > 
> > 
> > I guess we can do this in open() but then we will have to add again that
> > struct that wraps some ptdump specific state tracking. It seemed a bit cleaner in
> > this way. What do you think ?
> 
> Allocating something that looks like an iterator end embedding it in
> ->private isn't too uncommon.
> 

Ack, will stick to this approach in this case.

> > > >  	write_lock(&guest_kvm->mmu_lock);
> > > >  	ret = kvm_ptdump_show_common(m, mmu->pgt, &parser_state);
> > > >  	write_unlock(&guest_kvm->mmu_lock);
> > > >  
> > > > +	kvm_ptdump_parser_teardown(&parser_state);
> > > 
> > > Same question here, can this happen at close()? I guess you'll need a
> > > struct to encapsulate pg_state and a pointer to the VM at least.
> > >
> > 
> > Right, I tried to avoid using a separate struct as we discussed in v4.
> 
> Sorry, I hope I didn't confuse you in my prior feedback.
> 
> What I had issue with was the multiple layers of function ptr / ops
> structs for managing the file interface. I have zero concerns with
> organizing the _data_ for the walk this way.
>

Right, I see what you mean, thanks for the clarification.

> > > Actually, come to think of it, if you embed all of the data you need for
> > > the walker into a structure you can just do a single allocation for it
> > > upfront.
> 
> -- 
> Thanks,
> Oliver

Thanks for the feedback,
Seb