Nicholas Piggin <npig...@gmail.com> writes:
> PR KVM does not support running with AIL enabled, and SCV does is not
> supported with AIL disabled.
>
> Fix this by ensuring the SCV facility is disabled with FSCR while a
> CPU can be running with AIL=0. PowerNV host supports disabling AIL on a
> per-CPU basis, so SCV just needs to be disabled when a vCPU is run.
>
> The pSeries machine can only switch AIL on a system-wide basis, so it
> must disable SCV support at boot if the configuration can potentially
> run a PR KVM guest.
>
> SCV is not emulated for the PR guest at the moment, this just fixes the
> host crashes.
>
> Alternatives considered and rejected:
> - SCV support can not be disabled by PR KVM after boot, because it is
> advertised to userspace with HWCAP.
> - AIL can not be disabled on a per-CPU basis. At least when running on
> pseries it is a per-LPAR setting.
> - Support for real-mode SCV vectors will not be added because they are
> at 0x17000 so making such a large fixed head space causes immediate
> value limits to be exceeded, requiring a lot rework and more code.
> - Disabling SCV for any PR KVM possible kernel will cause a slowdown
> when not using PR KVM.
> - A boot time option to disable SCV to use PR KVM is user-hostile.
> - System call instruction emulation for SCV facility unavailable
> instructions is too complex and old emulation code was subtly broken
> and removed.
>
> Signed-off-by: Nicholas Piggin <npig...@gmail.com>
> ---
> arch/powerpc/kernel/exceptions-64s.S | 4 ++++
> arch/powerpc/kernel/setup_64.c | 15 +++++++++++++++
> arch/powerpc/kvm/book3s_pr.c | 20 ++++++++++++++------
> 3 files changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/kernel/exceptions-64s.S
> b/arch/powerpc/kernel/exceptions-64s.S
> index 55caeee37c08..b66dd6f775a4 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -809,6 +809,10 @@ __start_interrupts:
> * - MSR_EE|MSR_RI is clear (no reentrant exceptions)
> * - Standard kernel environment is set up (stack, paca, etc)
> *
> + * KVM:
> + * These interrupts do not elevate HV 0->1, so HV is not involved. PR KVM
> + * ensures that FSCR[SCV] is disabled whenever it has to force AIL off.
> + *
> * Call convention:
> *
> * syscall register convention is in Documentation/powerpc/syscall64-abi.rst
> diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
> index be8577ac9397..ac52c69a3811 100644
> --- a/arch/powerpc/kernel/setup_64.c
> +++ b/arch/powerpc/kernel/setup_64.c
> @@ -197,6 +197,21 @@ static void __init configure_exceptions(void)
>
> /* Under a PAPR hypervisor, we need hypercalls */
> if (firmware_has_feature(FW_FEATURE_SET_MODE)) {
> + /*
> + * PR KVM does not support AIL mode interrupts in the host, and
> + * SCV system call interrupt vectors are only implemented for
> + * AIL mode. Under pseries, AIL mode can only be enabled and
> + * disabled system-wide so when PR KVM is loaded, all CPUs in
> + * the host are set to AIL=0 mode. SCV can not be disabled
> + * dynamically because the feature is advertised to host
> + * userspace, so SCV support must not be enabled if PR KVM can
> + * possibly be run.
> + */
> + if (IS_ENABLED(CONFIG_KVM_BOOK3S_PR_POSSIBLE) &&
> !radix_enabled()) {
> + init_task.thread.fscr &= ~FSCR_SCV;
> + cur_cpu_spec->cpu_user_features2 &= ~PPC_FEATURE2_SCV;
> + }
> +
"Under pseries, AIL mode can only be enabled and disabled system-wide so
when PR KVM is loaded, all CPUs in the host are set to AIL=0 mode."
Loaded as in 'modprobe kvm_pr'? And host as in "nested host"
surely. Unless I completely misunderstood the patch (likely).
Is there a way to make this less unexpected to users? Maybe a few words
in the Kconfig entry for PR_POSSIBLE saying "if you enable this and run
a Hash MMU guest, you lose SCV"?
> /* Enable AIL if possible */
> if (!pseries_enable_reloc_on_exc()) {
> init_task.thread.fscr &= ~FSCR_SCV;
> diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
> index 34a801c3604a..4d1c84b94b77 100644
> --- a/arch/powerpc/kvm/book3s_pr.c
> +++ b/arch/powerpc/kvm/book3s_pr.c
> @@ -140,9 +140,12 @@ static void kvmppc_core_vcpu_load_pr(struct kvm_vcpu
> *vcpu, int cpu)
> #endif
>
> /* Disable AIL if supported */
> - if (cpu_has_feature(CPU_FTR_HVMODE) &&
> - cpu_has_feature(CPU_FTR_ARCH_207S))
> - mtspr(SPRN_LPCR, mfspr(SPRN_LPCR) & ~LPCR_AIL);
> + if (cpu_has_feature(CPU_FTR_HVMODE)) {
> + if (cpu_has_feature(CPU_FTR_ARCH_207S))
> + mtspr(SPRN_LPCR, mfspr(SPRN_LPCR) & ~LPCR_AIL);
> + if (cpu_has_feature(CPU_FTR_ARCH_300) && (current->thread.fscr
> & FSCR_SCV))
> + mtspr(SPRN_FSCR, mfspr(SPRN_FSCR) & ~FSCR_SCV);
> + }
>
> vcpu->cpu = smp_processor_id();
> #ifdef CONFIG_PPC_BOOK3S_32
> @@ -175,9 +178,12 @@ static void kvmppc_core_vcpu_put_pr(struct kvm_vcpu
> *vcpu)
> kvmppc_save_tm_pr(vcpu);
>
> /* Enable AIL if supported */
> - if (cpu_has_feature(CPU_FTR_HVMODE) &&
> - cpu_has_feature(CPU_FTR_ARCH_207S))
> - mtspr(SPRN_LPCR, mfspr(SPRN_LPCR) | LPCR_AIL_3);
> + if (cpu_has_feature(CPU_FTR_HVMODE)) {
> + if (cpu_has_feature(CPU_FTR_ARCH_207S))
> + mtspr(SPRN_LPCR, mfspr(SPRN_LPCR) | LPCR_AIL_3);
> + if (cpu_has_feature(CPU_FTR_ARCH_300) && (current->thread.fscr
> & FSCR_SCV))
> + mtspr(SPRN_FSCR, mfspr(SPRN_FSCR) | FSCR_SCV);
> + }
>
> vcpu->cpu = -1;
> }
> @@ -1037,6 +1043,8 @@ static int kvmppc_handle_fac(struct kvm_vcpu *vcpu,
> ulong fac)
>
> void kvmppc_set_fscr(struct kvm_vcpu *vcpu, u64 fscr)
> {
> + if (fscr & FSCR_SCV)
> + fscr &= ~FSCR_SCV; /* SCV must not be enabled */
> if ((vcpu->arch.fscr & FSCR_TAR) && !(fscr & FSCR_TAR)) {
> /* TAR got dropped, drop it in shadow too */
> kvmppc_giveup_fac(vcpu, FSCR_TAR_LG);
