> -----Original Message----- > From: Thomas Gleixner [mailto:t...@linutronix.de] > Sent: Monday, February 21, 2011 6:03 AM > To: KY Srinivasan > Cc: gre...@suse.de; linux-ker...@vger.kernel.org; > de...@linuxdriverproject.org; virtualizat...@lists.osdl.org; Haiyang Zhang; > Hank > Janssen > Subject: RE: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically > > On Mon, 21 Feb 2011, KY Srinivasan wrote: > > > > Like most virtualization platforms, Hyper-V also emulates the full PC > > > > platform. So, it is possible that the driver of some other emulated > > > > devices might register for the IRQ line we might have selected. That > > > > is the race this code addresses. For performance reasons, we want > > > > both storage and network traffic to go over the PV drivers. > > > > > > So in case your driver gets the interrupt line first, which the other > > > driver wants to acquire as well, then what? Do you want to do that > > > probe magic in the other driver as well? What if this is a regular > > > device driver which gets its irq number from ACPI/PCI or > > > whatever. Then that driver simply wont work as it's interrupt line is > > > busy. > > > > > > > > > > > > > I don't know why the previous reviewer wanted to have that > > > > > dynamic. That just does not make sense to me. > > > > > > > > Prior to this patch, we had a hard coded interrupt line for use by > > > > this driver. If that line was already in use, the load of this driver > > > > would fail. This would be a fatal issue especially for distributions > > > > that have embedded these PV drivers as part of their installation > > > > media. This patch deals with such collisions in a more graceful way - > > > > we would not bail until we have scanned all low interrupt lines. > > > > > > So you trade breaking the PV stuff against breaking random other > > > drivers? That doesn't sound like a brilliant idea. > > > > > > There are various ways to solve that proper. > > > > > > - You can provide the interrupt number from ACPI/PCI or whatever your HV > > > provides as enumeration. > > > > > > - Use a fixed vector like XEN does for the event channel > > > > > > - Use dynamic allocation in the IOAPIC space like the kernel does for > > > MSI(-X) > > > > > > Thanks, > > > > > > tglx > > > > I am not claiming that what I have done here is the best possible solution. > > However, I will submit to you that it is better than what we had here > > prior to this patch. I will address this and a whole lot of other issues > > in future patches. > > No, it's _NOT_ better in any way. You trade breaking your PV thing > against breaking random other drivers. Care to explain why you think > that's better ?
The root device for the VM is bound to the PV driver on some distributions. So, if we cannot load the PV drivers, we do not have a system that boots. In general, the system performance without these PV drivers is so poor that for all practical purposes, having the PV drivers is almost a requirement for having a useable system. While the platform supports configuration of the VM with some emulated devices, it is not a recommended configuration (because of performance reasons) for Linux virtual machines on the Hyper-V platform. I have spent significantly more time debating this patch than developing this patch that I still think improves the current driver. I will leave it to Greg and other powers that be to decide if this patch will be accepted. Let me know what your verdict is. Regards, K. Y _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linux-foundation.org/mailman/listinfo/virtualization
