From mboxrd@z Thu Jan 1 00:00:00 1970 From: Julien Grall Subject: Re: [linux-linus test] 30356: regressions - FAIL Date: Wed, 24 Sep 2014 00:35:05 +0100 Message-ID: <542203A9.3090702@linaro.org> References: <1411472727.14989.102.camel@kazak.uk.xensource.com> <54218FA7.8090706@linaro.org> <5421AE78.4040206@linaro.org> <1411497623.3824.1.camel@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1411497623.3824.1.camel@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Ian Campbell Cc: xen-devel@lists.xensource.com, "xen.org" , Stefano Stabellini List-Id: xen-devel@lists.xenproject.org On 23/09/2014 19:40, Ian Campbell wrote: > On Tue, 2014-09-23 at 18:31 +0100, Julien Grall wrote: >> I guess we will have to select LPAE when XEN is enabled, right? If >> it's the case that would mean the user won't be able to compile a >> Linux guest with short page table and Xen. >> >> Any though? > > Things must work without in guest LPAE too, so something somewhere else > will need fixing. > > Apart from restricting the user in an unwanted way requiring LPAE will > mean that practically no distro installer will work in a Xen guest. Xen does an identity mapping for the host physical address into DOM0 for the grant mapping. DOM0 will use a scratch page (see commit 340720b "xen/arm: reimplement xen_dma_unmap_page & friends") and map and this physical address. That means on platform with an address space higher than 32 bits, which is the case on Midway, we have to handle 64 bits physical address in DOM0. With the current implementation in Linux we can only use LPAE when a guest is started. The distro installer will still be able to work with short page table. The drawback is we are requiring LPAE from DOM0 and a different kernel in the guest if the user doesn't want to use LPAE. As the code is already pushed in Linux 3.17, I don't find a simpler solution to fix Linux boot without requiring LPAE. Regards, -- Julien Grall