Re: [PATCH][kvmtool] kvm: Request VM specific limits instead of system-wide ones

[Marc Zyngier <maz@kernel.org>]

On Mon, 27 Apr 2020 16:00:58 +0100
Alexandru Elisei <alexandru.elisei@arm.com> wrote:

> Hi,
> 
> On 4/27/20 3:44 PM, Alexandru Elisei wrote:
> > Hi,
> >
> > On 4/27/20 3:17 PM, Marc Zyngier wrote:  
> >> On arm64, the maximum number of vcpus is constrained by the type
> >> of interrupt controller that has been selected (GICv2 imposes a
> >> limit of 8 vcpus, while GICv3 currently has a limit of 512).
> >>
> >> It is thus important to request this limit on the VM file descriptor
> >> rather than on the one that corresponds to /dev/kvm, as the latter
> >> is likely to return something that doesn't take the constraints into
> >> account.
> >>
> >> Reported-by: Ard Biesheuvel <ardb@kernel.org>
> >> Signed-off-by: Marc Zyngier <maz@kernel.org>
> >> ---
> >>  kvm.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kvm.c b/kvm.c
> >> index e327541..3d5173d 100644
> >> --- a/kvm.c
> >> +++ b/kvm.c
> >> @@ -406,7 +406,7 @@ int kvm__recommended_cpus(struct kvm *kvm)
> >>  {
> >>  	int ret;
> >>  
> >> -	ret = ioctl(kvm->sys_fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
> >> +	ret = ioctl(kvm->vm_fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
> >>  	if (ret <= 0)
> >>  		/*
> >>  		 * api.txt states that if KVM_CAP_NR_VCPUS does not exist,
> >> @@ -421,7 +421,7 @@ int kvm__max_cpus(struct kvm *kvm)
> >>  {
> >>  	int ret;
> >>  
> >> -	ret = ioctl(kvm->sys_fd, KVM_CHECK_EXTENSION, KVM_CAP_MAX_VCPUS);
> >> +	ret = ioctl(kvm->vm_fd, KVM_CHECK_EXTENSION, KVM_CAP_MAX_VCPUS);
> >>  	if (ret <= 0)
> >>  		ret = kvm__recommended_cpus(kvm);
> >>    
> > I've checked that gic__create comes before the call kvm__recommended_capus:
> > gic__create is in core_init (called via kvm__init->kvm_arch_init), and
> > kvm__recommended_cpus is in base_init (called via kvm__cpu_init ->
> > kvm__{recommended,max}_cpus).
> >
> > The KVM api documentation states that KVM_CHECK_EXTENSION is available for the vm
> > fd only if the system capability KVM_CAP_CHECK_EXTENSION_VM is present. kvmtool
> > already has a function for checking extensions on the vm fd, it's called
> > kvm__supports_vm_extension. Can we use that instead of doing the ioctl directly on
> > the vm fd?  
> 
> Scratch that, kvm__supports_vm_extension returns a bool, not an int.
> How about we write kvm__check_vm_extension that returns an int, and
> kvm__supports_vm_extension calls it?

That, or we just change the return type for kvm__supports_vm_extension,
and hack the only places that uses it so far (the GIC code) to detect
the error.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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