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Date:   Wed, 12 Oct 2022 10:26:19 +0300
From:   Jarkko Sakkinen <jarkko@kernel.org>
To:     Ira Weiny <ira.weiny@intel.com>
Cc:     Kristen Carlson Accardi <kristen@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-sgx@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] x86/sgx: Replace kmap/kunmap_atomic calls
Message-ID: <Y0ZsG2xhiqcGc7al@kernel.org>
References: <20220929160647.362798-1-kristen@linux.intel.com>
 <Y0BEV+Xgkrln8xoh@iweiny-desk3>
 <Y0ZphugZZBhlv/vT@kernel.org>
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On Wed, Oct 12, 2022 at 10:15:22AM +0300, Jarkko Sakkinen wrote:
> On Fri, Oct 07, 2022 at 08:23:03AM -0700, Ira Weiny wrote:
> > On Thu, Sep 29, 2022 at 09:06:46AM -0700, Kristen Carlson Accardi wrote:
> > > It is not necessary to disable page faults or preemption when
> > > using kmap calls, so replace kmap_atomic() and kunmap_atomic()
> > > calls with more the more appropriate kmap_local_page() and
> > > kunmap_local() calls.
> > > 
> > > Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> > 
> > Minor comment below.
> > 
> > > ---
> > >  arch/x86/kernel/cpu/sgx/encl.c  | 12 ++++++------
> > >  arch/x86/kernel/cpu/sgx/ioctl.c |  4 ++--
> > >  arch/x86/kernel/cpu/sgx/main.c  |  8 ++++----
> > >  3 files changed, 12 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> > > index f40d64206ded..63dd92bd3288 100644
> > > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > > @@ -160,8 +160,8 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
> > >  		return ret;
> > >  
> > >  	pginfo.addr = encl_page->desc & PAGE_MASK;
> > > -	pginfo.contents = (unsigned long)kmap_atomic(b.contents);
> > > -	pcmd_page = kmap_atomic(b.pcmd);
> > > +	pginfo.contents = (unsigned long)kmap_local_page(b.contents);
> > > +	pcmd_page = kmap_local_page(b.pcmd);
> > >  	pginfo.metadata = (unsigned long)pcmd_page + b.pcmd_offset;
> > >  
> > >  	if (secs_page)
> > > @@ -187,8 +187,8 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
> > >  	 */
> > >  	pcmd_page_empty = !memchr_inv(pcmd_page, 0, PAGE_SIZE);
> > >  
> > > -	kunmap_atomic(pcmd_page);
> > > -	kunmap_atomic((void *)(unsigned long)pginfo.contents);
> > > +	kunmap_local(pcmd_page);
> > > +	kunmap_local((void *)(unsigned long)pginfo.contents);
> > >  
> > >  	get_page(b.pcmd);
> > >  	sgx_encl_put_backing(&b);
> > > @@ -197,10 +197,10 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
> > >  
> > >  	if (pcmd_page_empty && !reclaimer_writing_to_pcmd(encl, pcmd_first_page)) {
> > >  		sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
> > > -		pcmd_page = kmap_atomic(b.pcmd);
> > > +		pcmd_page = kmap_local_page(b.pcmd);
> > >  		if (memchr_inv(pcmd_page, 0, PAGE_SIZE))
> > >  			pr_warn("PCMD page not empty after truncate.\n");
> > > -		kunmap_atomic(pcmd_page);
> > > +		kunmap_local(pcmd_page);
> > >  	}
> > >  
> > >  	put_page(b.pcmd);
> > > diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> > > index ebe79d60619f..f2f918b8b9b1 100644
> > > --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> > > +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> > > @@ -221,11 +221,11 @@ static int __sgx_encl_add_page(struct sgx_encl *encl,
> > >  	pginfo.secs = (unsigned long)sgx_get_epc_virt_addr(encl->secs.epc_page);
> > >  	pginfo.addr = encl_page->desc & PAGE_MASK;
> > >  	pginfo.metadata = (unsigned long)secinfo;
> > > -	pginfo.contents = (unsigned long)kmap_atomic(src_page);
> > > +	pginfo.contents = (unsigned long)kmap_local_page(src_page);
> > >  
> > >  	ret = __eadd(&pginfo, sgx_get_epc_virt_addr(epc_page));
> > >  
> > > -	kunmap_atomic((void *)pginfo.contents);
> > > +	kunmap_local((void *)pginfo.contents);
> > >  	put_page(src_page);
> > >  
> > >  	return ret ? -EIO : 0;
> > > diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> > > index 515e2a5f25bb..4efda5e8cadf 100644
> > > --- a/arch/x86/kernel/cpu/sgx/main.c
> > > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > > @@ -159,17 +159,17 @@ static int __sgx_encl_ewb(struct sgx_epc_page *epc_page, void *va_slot,
> > >  	pginfo.addr = 0;
> > >  	pginfo.secs = 0;
> > >  
> > > -	pginfo.contents = (unsigned long)kmap_atomic(backing->contents);
> > > -	pginfo.metadata = (unsigned long)kmap_atomic(backing->pcmd) +
> > > +	pginfo.contents = (unsigned long)kmap_local_page(backing->contents);
> > > +	pginfo.metadata = (unsigned long)kmap_local_page(backing->pcmd) +
> > >  			  backing->pcmd_offset;
> > >  
> > >  	ret = __ewb(&pginfo, sgx_get_epc_virt_addr(epc_page), va_slot);
> > >  	set_page_dirty(backing->pcmd);
> > >  	set_page_dirty(backing->contents);
> > >  
> > > -	kunmap_atomic((void *)(unsigned long)(pginfo.metadata -
> > > +	kunmap_local((void *)(unsigned long)(pginfo.metadata -
> > >  					      backing->pcmd_offset));
> > 
> > For kunmap_local() one can use any address in the page.  So this can be:
> > 
> > 	kunmap_local((void *)(unsigned long)(pginfo.metadata));
> > 
> > 
> > Regardless:
> > 
> > Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> 
> There's no data to show that this change would be useful to do.
> 
> Thus, as said earlier, the commit message is missing "why".
> 
> Definitive NAK with the current offering.

Concurrency is complex enough topic that it is pretty hard to predict
the difference without any data. Any sort of benchmark, workload or
whatever to support the change would be essential here.

If we change primitives with weak arguments, it might backlash with
someone using SGX complaining about degrade in performance in some use
case.

*Conceptually* I do not have anything against this change but it is not
good enough argument here.

BR, Jarkko