One more bump:) MW..
On Wed, Jun 4, 2014 at 9:42 PM, Marcus White <[email protected]> wrote: > Hello > Just a bump to see if anyone can help:) > > MW > > On Tue, Jun 3, 2014 at 6:08 PM, Marcus White <[email protected]> > wrote: >> Cant the pages be locked down by the host when it gets the call down >> for an operation with a buffer(like if a usual user mode process >> case), and once locked down in the host kernel, the physical address >> be used directly for DMA, whether send or recieve? Not sure if I fully >> understand why a copy will be done irrespective. Can you please >> clarify? >> >> Also in the QEMU case, since they are all part of the same process (VM >> and QEMU), where is the data copy? >> >> Apologies if these are basic questions, trying to understand the path here. >> >> Thanks in advance. >> MW >> >> >> >> On Tue, Jun 3, 2014 at 4:25 AM, Venkateswara Rao Nandigam >> <[email protected]> wrote: >>>>>>> I know that kvm support network tx zero-copy when using vhost, but rx >>>>>>> copy is still performed in vhost, because the NIC cannot determine DMA >>>>>>> to which VM's rx buffers before L2 switching(unless vhost using >>>>>>> page-flip between HVA->HPA and GPA->HPA, or macvtap over SRIOV-VF is >>>>>>> used). >>>>>>>>Storage has no this limitation, can vhost-blk and vhost-scsi avoid data >>>>>>>>copy in both write and read? >>> >>> A copy in kernel will be done irrespective of Rx/Tx when vhost is used. One >>> copy between user space and kernel space in QEMU is avoided when vhost is >>> used. That is why using vhost is "realtively" zero copy. >>> >>> I don’t think that they can be classified as vhost-blk or vhost-scsi. Vhost >>> is an add on to an existing Guest driver, like virtio. So they can be >>> called virtio-blk, virtio-scsi >>> >>> So, using PV drivers(virtio-blk) for disk access makes accesses faster and >>> using vhost on top of this will make it much more faster as one copy will >>> be avoided. >>> >>> Linus AIO will facilitate avoiding blocking on a i/o, but doubt if it has >>> anything to do with copy across user space and kernel space. >>> >>> -----Original Message----- >>> From: [email protected] [mailto:[email protected]] On >>> Behalf Of Zhang Haoyu >>> Sent: Tuesday, June 03, 2014 4:30 PM >>> To: Venkateswara Rao Nandigam; Marcus White >>> Cc: kvm >>> Subject: Re: RE: Some more basic questions.. >>> >>>>>A few additional questions:) >>>>> >>>>>1. If IO to a block device goes through QEMU and not vhost, are there >>>>>data copies between kernel and user mode if I do IO to a block device >>>>>or is it zero copy? Kind of related to Question (2) also. >>>>> >>>>An additional copy will be avoided only by using vhost, so if you are using >>>>vhost you can call it zero copy realtively. >>> I know that kvm support network tx zero-copy when using vhost, but rx copy >>> is still performed in vhost, because the NIC cannot determine DMA to which >>> VM's rx buffers before L2 switching(unless vhost using page-flip between >>> HVA->HPA and GPA->HPA, or macvtap over SRIOV-VF is used). >>> Storage has no this limitation, can vhost-blk and vhost-scsi avoid data >>> copy in both write and read? >>> >>> IIUC, even if vhost is not used, qemu using linux native aio can avoid data >>> copy between user and kernel space, right? >>> >>> -- >>> To unsubscribe from this list: send the line "unsubscribe kvm" in the body >>> of a message to [email protected] More majordomo info at >>> http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
