From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <47E2CAAC.6020903@de.ibm.com> Date: Thu, 20 Mar 2008 21:35:56 +0100 From: Carsten Otte Reply-To: carsteno@de.ibm.com MIME-Version: 1.0 Subject: Re: [kvm-devel] [RFC/PATCH 01/15] preparation: provide hook to enable pgstes in user pagetable References: <1206028710.6690.21.camel@cotte.boeblingen.de.ibm.com> <1206030278.6690.52.camel@cotte.boeblingen.de.ibm.com> <47E29EC6.5050403@goop.org> <1206040405.8232.24.camel@nimitz.home.sr71.net> In-Reply-To: <1206040405.8232.24.camel@nimitz.home.sr71.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Dave Hansen Cc: Jeremy Fitzhardinge , Christian Ehrhardt , hollisb@us.ibm.com, arnd@arndb.de, borntrae@linux.vnet.ibm.com, kvm-devel@lists.sourceforge.net, heicars2@linux.vnet.ibm.com, jeroney@us.ibm.com, Avi Kivity , virtualization@lists.linux-foundation.org, Linux Memory Management List , mschwid2@linux.vnet.ibm.com, rvdheij@gmail.com, Olaf Schnapper , jblunck@suse.de, "Zhang, Xiantao" List-ID: Dave Hansen wrote: > Well, and more fundamentally: do we really want dup_mm() able to be > called from other code? > > Maybe we need a bit more detailed justification why fork() itself isn't > good enough. It looks to me like they basically need an arch-specific > argument to fork, telling the new process's page tables to take the > fancy new bit. > > I'm really curious how this new stuff is going to get used. Are you > basically replacing fork() when creating kvm guests? No. The trick is, that we do need bigger page tables when running guests: our page tables are usually 2k, but when running a guest they're 4k to track both guest and host dirty&reference information. This looks like this: *----------* *2k PTE's * *----------* *2k PGSTE * *----------* We don't want to waste precious memory for all page tables. We'd like to have one kernel image that runs regular server workload _and_ guests. Therefore, we need to reallocate the page table after fork() once we know that task is going to be a hypervisor. That's what this code does: reallocate a bigger page table to accomondate the extra information. The task needs to be single-threaded when calling for extended page tables. Btw: at fork() time, we cannot tell whether or not the user's going to be a hypervisor. Therefore we cannot do this in fork. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org