From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1NjxVc-0005Ln-Jt for qemu-devel@nongnu.org; Tue, 23 Feb 2010 11:21:16 -0500 Received: from [199.232.76.173] (port=54949 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NjxVc-0005Ki-3x for qemu-devel@nongnu.org; Tue, 23 Feb 2010 11:21:16 -0500 Received: from Debian-exim by monty-python.gnu.org with spam-scanned (Exim 4.60) (envelope-from ) id 1NjxVa-000630-RO for qemu-devel@nongnu.org; Tue, 23 Feb 2010 11:21:16 -0500 Received: from mail-yx0-f204.google.com ([209.85.210.204]:44124) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1NjxVa-00062U-I1 for qemu-devel@nongnu.org; Tue, 23 Feb 2010 11:21:14 -0500 Received: by yxe42 with SMTP id 42so388773yxe.4 for ; Tue, 23 Feb 2010 08:21:08 -0800 (PST) Message-ID: <4B84006F.7050406@codemonkey.ws> Date: Tue, 23 Feb 2010 10:21:03 -0600 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Address translation - virt->phys->ram References: <4B828DC2.3000609@collabora.co.uk> <4B829645.5020904@codemonkey.ws> <4B82B4E3.2040805@collabora.co.uk> <4B82B655.2000408@codemonkey.ws> <4B82C325.5020300@collabora.co.uk> In-Reply-To: <4B82C325.5020300@collabora.co.uk> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Ian Molton Cc: qemu-devel@nongnu.org On 02/22/2010 11:47 AM, Ian Molton wrote: > Anthony Liguori wrote: > >> On 02/22/2010 10:46 AM, Ian Molton wrote: >> >>> Anthony Liguori wrote: >>> >>> >>> >>>> cpu_physical_memory_map(). >>>> >>>> But this function has some subtle characteristics. It may return a >>>> bounce buffer if you attempt to map MMIO memory. There is a limited >>>> pool of bounce buffers available so it may return NULL in the event that >>>> it cannot allocate a bounce buffer. >>>> >>>> It may also return a partial result if you're attempting to map a region >>>> that straddles multiple memory slots. >>>> >>>> >>> Thanks. I had found this, but was unsure as to wether it was quite what >>> I wanted. (also is it possible to tell when it has (eg.) allocated a >>> bounce buffer?) >>> >>> Basically, I need to get buffer(s) from guest userspace into the hosts >>> address space. The buffers are virtually contiguous but likely >>> physically discontiguous. They are allocated with malloc() and theres >>> nothing I can do about that. >>> >>> The obvious but slow solution would be to copy all the buffers into nice >>> virtio-based scatter/gather buffers and feed them to the host that way, >>> however its not fast enough. >>> >>> >> Why is this slow? >> > Because the buffers will all have to be copied. Why? It sounds like your kernel driver is doing the wrong thing if you can't preserve zero-copy from userspace. > So far, switching from > abusing an instruction to interrupt qemu to using virtio has incurred a > roughly 5x slowdown. If you post patches, we can help determine what you're doing that's causing such a slow down. Regards, Anthony Liguori > I'd guess much of this is down to the fact we have > to switch to kernel-mode on the guest and back again for every single GL > call... > > If I can establish some kind of stable guest_virt->phys->host_virt > mapping, many of the problems will just 'go away'. a way to interrupt > qemu from user-mode on the guest without involving the guest kernel > would be quite awesome also (theres really nothing we want the kernel to > actually /do/ here, it just adds overhead). > > -Ian > >