From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42780) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ebk2Y-0006kr-B1 for qemu-devel@nongnu.org; Wed, 17 Jan 2018 04:29:51 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ebk2V-0006dG-4D for qemu-devel@nongnu.org; Wed, 17 Jan 2018 04:29:50 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38084) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ebk2U-0006c7-Tf for qemu-devel@nongnu.org; Wed, 17 Jan 2018 04:29:47 -0500 References: <20180114090147.39255-1-marcel@redhat.com> <20180114090147.39255-4-marcel@redhat.com> <20180116040030-mutt-send-email-mst@kernel.org> From: Marcel Apfelbaum Message-ID: <6aa6ebfb-1f45-7647-883a-58b81ececc7d@redhat.com> Date: Wed, 17 Jan 2018 11:29:24 +0200 MIME-Version: 1.0 In-Reply-To: <20180116040030-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH V7 3/5] docs: add pvrdma device documentation. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, ehabkost@redhat.com, imammedo@redhat.com, yuval.shaia@oracle.com, pbonzini@redhat.com, borntraeger@de.ibm.com, cohuck@redhat.com, f4bug@amsat.org Hi Michael, On 16/01/2018 4:06, Michael S. Tsirkin wrote: > On Sun, Jan 14, 2018 at 11:01:45AM +0200, Marcel Apfelbaum wrote: >> +5. Limitations >> +============== > > Limitations are fine but need to cause init failures since users > don't poke in the internal documentation. > Good point, thanks. >> +- The device obviously is limited by the Guest Linux Driver features implementation >> + of the VMware device API. >> +- Memory registration mechanism requires mremap for every page in the buffer in order >> + to map it to a contiguous virtual address range. Since this is not the data path >> + it should not matter much. > > Depends on the size of the region I guess. Did you try this with several > gigabytes of memory? On a "standard" laptop it takes about 0.3 secs for 0.5G and about 0.5 secs for 1G. There is no impact for memory regions less than 0.5G. > If we are talking seconds of downtime, > it's worth documenting so people aren't surprised. > Alternatively, limit the max size of MR? We already limited the MAX MR to ~ 134MB. (1<<27), however the user can change the parameter at command line. I will add to documentation to pay attention than using 1G MRs can take half second to register. > >> +- The device requires target page size to be the same as the host page size. > > Pls add code to fail init when this is not the case. > Sure >> +- QEMU cannot map guest RAM from a file descriptor if a pvrdma device is attached, >> + so it can't work with huge pages. The limitation will be addressed in the future, >> + however QEMU allocates Guest RAM with MADV_HUGEPAGE so if there are enough huge >> + pages available, QEMU will use them. > > Same here. > Will add for next re-spin. >> +- As previously stated, migration is not supported yet, however with some hardware >> + support can be done. > > I don't see a migration blocker. > Will remove from the documentation, thanks. Thank you for the review, Marcel > >