linux-kernel - Re: [PATCH v4 1/3] x86/sgx: Use sgx_free_epc_page() in sgx_reclaim

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Date:   Tue, 16 Mar 2021 14:50:04 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-sgx@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] x86/sgx: Use sgx_free_epc_page() in
 sgx_reclaim_pages()

On Mon, Mar 15, 2021 at 09:27:00PM +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 15, 2021 at 09:06:29PM +0200, Jarkko Sakkinen wrote:
> > On Mon, Mar 15, 2021 at 08:32:13AM -0700, Dave Hansen wrote:
> > > On 3/13/21 8:01 AM, Jarkko Sakkinen wrote:
> > > > Replace the ad-hoc code with a sgx_free_epc_page(), in order to make sure
> > > > that all the relevant checks and book keeping is done, while freeing a
> > > > borrowed EPC page, and remove redundant code. EREMOVE inside
> > > > sgx_free_epc_page() does not change the semantics, as EREMOVE to an
> > > > uninitialize pages is a nop.
> > > 
> > >   ^ uninitialized
> > > 
> > > I know this is a short patch, but this changelog still falls a bit short
> > > for me.
> > > 
> > > Why is this patch a part of _this_ series?  What *problem* does it
> > > solve, related to this series?
> > 
> > I'm thinking of merging sgx_epc_section and sgx_numa_node. That's why I
> > kept it as part of the series. 
> > 
> > Also, in any case it's better to clean up duplicate functionality. The
> > code is essentially open coded implementation of sgx_free_epc_page()
> > without EREMOVE.
> > 
> > > It would also be nice to remind me why the EREMOVE is redundant.  Why
> > > didn't we need one before?  What put the page in the uninitialized
> > > state?  Is EREMOVE guaranteed to do nothing?  How expensive is it?
> > 
> > EREMOVE gets removed by KVM series from sgx_free_epc_page() anyway.
> > 
> > Maybe should re-send this patch, or series, after KVM series is merged.
> > Then there is no explaining with EREMOVE, as sgx_free_epc_page() won't
> > contain it.
> 
> Anyway, forgot to put the end statement: I'm cool with dropping this but
> I'll also send this right after KVM SGX series has landed as separate
> patch, if I drop this now.

HOLD ON :-)

I recalled why I added this patch to this patch set. I had a reason for
it.

It's because of the NUMA patch. I have duplicate all the NUMA changes
here if I don't refactor this somewhat redundant code out.

So, if I add a note about this to the commit message? IMHO, this is good
enough reason to carry the patch.

/Jarkko