linux-kernel - Re: [PATCH v38 16/24] x86/sgx: Add a page reclaimer

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Date:   Tue, 22 Sep 2020 20:02:03 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Jethro Beekman <jethro@...tanix.com>,
        Jordan Hand <jorhand@...ux.microsoft.com>,
        Nathaniel McCallum <npmccallum@...hat.com>,
        Chunyang Hui <sanqian.hcy@...fin.com>,
        Seth Moore <sethmo@...gle.com>, akpm@...ux-foundation.org,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        dave.hansen@...el.com, haitao.huang@...el.com,
        josh@...htriplett.org, 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
Subject: Re: [PATCH v38 16/24] x86/sgx: Add a page reclaimer

On Tue, Sep 22, 2020 at 09:24:38AM -0700, Sean Christopherson wrote:
> The "else if" is correct.  Version Array (VA) pages have 512 slots that hold
> metadata for evicted EPC pages, i.e. swapping a page out of the EPC requires
> a VA slot.  For simplicity (LOL),

I'll say.

> the approach we are taking for initial support is to reserve a VA slot
> when adding a page to the enclave[*]. In most cases, reserving a slot
> does not require allocating a new VA page, e.g. to reserve slots 1-511
> of the "current" VA page. The if-elif is handling the case where the
> current VA page is fully reserved and a new one needs to be allocated.
> The if handles the error, the elif handles success, i.e.
> 
> 	if (IS_ERR(va_page)) <- needed a new VA page, allocation failed
> 		return PTR_ERR(va_page);
> 	else if (va_page)    <- needed a new VA page, allocation succeeded
> 		list_add(&va_page->list, &encl->va_pages);
> 	else
> 		             <- reused the current VA page

Aha, in that case va_page will be NULL but you're not using it in that
function later and that current VA page is on encl->va_pages which
others can get from there. Ok, gotcha.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette