From: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Friday, February 16, 2024 6:10 AM > To: kys@xxxxxxxxxxxxx; haiyangz@xxxxxxxxxxxxx; wei.liu@xxxxxxxxxx; > decui@xxxxxxxxxxxxx; linux-hyperv@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx > Cc: ssengar@xxxxxxxxxxxxx > Subject: [PATCH] Drivers: hv: Kconfig: select CPUMASK_OFFSTACK for Hyper-V > > CPUMASK_OFFSTACK must be set to have NR_CPUS_RANGE_END value greater than > 512, which eventually allows NR_CPUS > 512. > > CPUMASK_OFFSTACK can also be enabled by setting MAXSMP=y, but that will > set NR_CPUS=8192. This is not accurate for Hyper-V, because maximum number > of vCPU supported by Hyper-V today is 2048. Thus, enabling MAXSMP increase > the vmlinux size unnecessary. Note that these statements apply only to x86. arm64 doesn't have MAXSMP or NR_CPUS_RANGE_END. > > This option allows NR_CPUS=2048 which saves around 1MB of vmlinux size > for Hyper-V. > > Signed-off-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> > --- > drivers/hv/Kconfig | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig > index 0024210..bc3f496 100644 > --- a/drivers/hv/Kconfig > +++ b/drivers/hv/Kconfig > @@ -9,6 +9,7 @@ config HYPERV > select PARAVIRT > select X86_HV_CALLBACK_VECTOR if X86 > select OF_EARLY_FLATTREE if OF > + select CPUMASK_OFFSTACK > help > Select this option to run Linux as a Hyper-V client operating > system. > -- > 1.8.3.1 > I'm not sure that enabling CPUMASK_OFFSTACK for Hyper-V guests is the right thing to do, as there's additional runtime cost when CPUMASK_OFFSTACK is enabled. I agree that for the most general case, you want NR_CPUS to be 2048, which requires CPUMASK_OFFSTACK. But it would be legitimate to build a kernel with NR_CPUS set to something like 64 or 256 for a more limited Hyper-V guest use case, and to not want to incur the cost of CPUMASK_OFFSTACK. You could consider doing something like this: select CPUMASK_OFFSTACK if NR_CPUS > 512 But kernel builders always have the option of explicitly enabling CPUMASK_OFFSTACK. That's what I see in the distro vendor arm64 images in Azure, since there's currently nothing that automatically selects CPUMASK_OFFSTACK for arm64. So I'm wondering if selecting CPUMASK_OFFSTACK under HYPERV should be added at all. The two aren't really related. There are recent LKML threads on enabling CPUMASK_OFFSTACK for arm64 -- see links below for some useful discussion of the topic in general. Michael [1] https://lore.kernel.org/lkml/794a1211-630b-3ee5-55a3-c06f10df1490@xxxxxxxxx/ [2] https://lore.kernel.org/lkml/7ab6660e-e69f-a64b-0de3-b8dde14f79fa@xxxxxxxxx/ [3] https://lore.kernel.org/lkml/e0d41efb-a74e-6bb5-f325-63d42358c802@xxxxxxxxxx/
