On Tue, Apr 4, 2023 at 6:45 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > > In the patch I send in last email, We speculate that the VMD > > Controller aggregate interrupts, > > making the PCIe port less stressed and improving iops. In this > > case(lots of 4k random IO), if we enable the VMD MSI > > remapping, we found the interrupts from VMD Controller's MSI are less > > and the IOPS is much higher. > > Great, that's useful information about why users would want to use > this parameter. > > Obviously the other half is to mention the reasons why they may not be > able to use it. If "msi_remap=off" were *always* safe and desirable, > we would just do that unconditionally and there would be no point in a > parameter. > > > > > I place the "vmd_config_msi_remap_param" that is VMD MSI-X's mode > > > > param configuring helper front > > > > "vmd_enable_domain". So, It will not change the logic disabling > > > > remapping from ee81ee84f873, such as > > > > "Currently MSI remapping must be enabled in guest passthrough mode". > > > > So, if the user config the wrong type, it will not work, and they can > > > > find it by dmesg. > > > > > > That's kind of a problem. I'm not in favor of something failing and > > > the user having to debug it via dmesg. That causes user frustration > > > and problem reports. > > > > What about adding a sysfs node for it in VMD PCI bus dir, which allows > > users to catch VMD's MSI current working mode? > > No, a sysfs node is not the answer to this. If we *can* avoid a > non-working situation, we should avoid it. If users see a non-working > situation, they will rightly complain, and somebody will have to debug > it. We don't want that. emm~ I privately suppose: In the traditional way without module parameters, this problem also exists. If the user disables remapping in guest os, the VMD driver will force it to remapping mode. What about I add the additional description in MODULE_PARM_DESC and comment? As for some devices not support disable remapping, What about changing the param to disabae_bypass=0/1? And make it clear in the description: this parameter will only affect the disabling of bypass mode on devices that support it. > > My point is that apparently guest passthrough mode is relevant and > maybe it should be part of the commit log about how this parameter > should be used. You are right~ I will add some clarification on how to configure VMD MSI in the guest. > > Bjorn
