Re: [RFC] powerpc/kvm: Fix spinlock member access for PREEMPT_RT

[Vishal Chourasia <vishalc@linux.ibm.com>]

On Thu, Oct 10, 2024 at 11:23:55PM +0200, Paolo Bonzini wrote:
> On 10/10/24 20:09, Vishal Chourasia wrote:
> > Hi,
> > 
> > While building the kernel with CONFIG_PREEMPT_RT, I encountered several
> > compilation errors in the PowerPC KVM code. The issues appear in
> > book3s_hv_rm_mmu.c where it tries to access the 'rlock' member of struct
> > spinlock, which doesn't exist in the RT configuration.
> 
> How was this tested? I suspect that putting to sleep a task that is running
> in real mode is a huge no-no.  The actual solution would have to be to split
> mmu_lock into a spin_lock and a raw_spin_lock, but that's a huge amount of
> work probably.  I'd just add a "depends on !PPC || !KVM_BOOK3S_64_HV" or
> something like that, to prevent enabling KVM-HV on PREEMPT_RT kernels.
Hi Paolo,

This is a build time error, I didn't boot the kernel.

I used pseries_le_defconfig with some other configs enabled. I was trying
to see if the kernel would compile with ARCH_SUPPORTS_RT and
CONFIG_PREEMPT_RT enabled.

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 8094a01974cca..568dc856f0dfa 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -168,6 +168,7 @@ config PPC
        select ARCH_STACKWALK
        select ARCH_SUPPORTS_ATOMIC_RMW
        select ARCH_SUPPORTS_DEBUG_PAGEALLOC    if PPC_BOOK3S || PPC_8xx
+       select ARCH_SUPPORTS_RT                 if !PPC || !KVM_BOOK3S_64_HV
        select ARCH_USE_BUILTIN_BSWAP
        select ARCH_USE_CMPXCHG_LOCKREF         if PPC64
        select ARCH_USE_MEMTEST
I tried rebuilding with the above diff as per your suggestion
though it works when KVM_BOOK3S_64_HV is set to N, but for 
pseries_le_defconfig, it's set to M, by default, which then requires setting it
to N explicitly.


Will something like below be a better solution? This will set
KVM_BOOK3S_64_HV to N if ARCH_SUPPORTS_RT is set.

diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
index dbfdc126bf144..33e0d50b08b14 100644
--- a/arch/powerpc/kvm/Kconfig
+++ b/arch/powerpc/kvm/Kconfig
@@ -80,7 +80,7 @@ config KVM_BOOK3S_64

 config KVM_BOOK3S_64_HV
        tristate "KVM for POWER7 and later using hypervisor mode in host"
-       depends on KVM_BOOK3S_64 && PPC_POWERNV
+       depends on KVM_BOOK3S_64 && PPC_POWERNV && !ARCH_SUPPORTS_RT
        select KVM_BOOK3S_HV_POSSIBLE
        select KVM_GENERIC_MMU_NOTIFIER
        select CMA

Thanks