linux-kernel - Re: [PATCH v39 11/24] x86/sgx: Add SGX enclave driver

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Message-ID: <20201005013053.GJ20115@casper.infradead.org>
Date:   Mon, 5 Oct 2020 02:30:53 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Chunyang Hui <sanqian.hcy@...fin.com>,
        Jordan Hand <jorhand@...ux.microsoft.com>,
        Nathaniel McCallum <npmccallum@...hat.com>,
        Seth Moore <sethmo@...gle.com>,
        Darren Kenny <darren.kenny@...cle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Suresh Siddha <suresh.b.siddha@...el.com>,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        dave.hansen@...el.com, haitao.huang@...el.com, kai.huang@...el.com,
        kai.svahn@...el.com, kmoy@...gle.com, ludloff@...gle.com,
        luto@...nel.org, nhorman@...hat.com, puiterwijk@...hat.com,
        rientjes@...gle.com, tglx@...utronix.de, yaozhangx@...gle.com,
        mikko.ylinen@...el.com
Subject: Re: [PATCH v39 11/24] x86/sgx: Add SGX enclave driver

On Mon, Oct 05, 2020 at 02:41:53AM +0300, Jarkko Sakkinen wrote:
> On Sun, Oct 04, 2020 at 11:27:50PM +0100, Matthew Wilcox wrote:
> > 	int ret = 0;
> > 
> > 	mutex_lock(&encl->lock);
> > 	rcu_read_lock();
> 
> Right, so xa_*() take RCU lock implicitly and xas_* do not.

Not necessarily the RCU lock ... I did document all this in xarray.rst:

https://www.kernel.org/doc/html/latest/core-api/xarray.html

> > 	while (xas.index < idx_end) {
> > 		page = xas_next(&xas);
> 
> It should iterate through every possible page index within the range,
> even the ones that do not have an entry, i.e. this loop also checks
> that there are no empty slots.
> 
> Does xas_next() go through every possible index, or skip the non-empty
> ones?

xas_next(), as its documentation says, will move to the next array
index:

https://www.kernel.org/doc/html/latest/core-api/xarray.html#c.xas_next

> > 		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
> > 			ret = -EACCESS;
> > 			break;
> > 		}
> > 	}
> > 	rcu_read_unlock();
> > 	mutex_unlock(&encl->lock);
> 
> In my Geminilake NUC the maximum size of the address space is 64GB for
> an enclave, and it is not fixed but can grow in microarchitectures
> beyond that.
> 
> That means that in (*artificial*) worst case the locks would be kept for
> 64*1024*1024*1024/4096 = 16777216 iterations.

Oh, there's support for that on the XArray API too.

        xas_lock_irq(&xas);
        xas_for_each_marked(&xas, page, end, PAGECACHE_TAG_DIRTY) {
                xas_set_mark(&xas, PAGECACHE_TAG_TOWRITE);
                if (++tagged % XA_CHECK_SCHED)
                        continue;

                xas_pause(&xas);
                xas_unlock_irq(&xas);
                cond_resched();
                xas_lock_irq(&xas);
        }
        xas_unlock_irq(&xas);