Re: [PATCH v2 1/2] KVM: PPC: Book3S PR: Disable SCV when AIL could be disabled

[Fabiano Rosas <farosas@linux.ibm.com>]

Nicholas Piggin <npiggin@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 could be running with AIL=0.
>
> The PowerNV host supports disabling AIL on a per-CPU basis, so SCV just
> needs to be disabled when a vCPU is being 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.
>
> Also ensure a the FSCR[SCV] bit can not be enabled when emulating
> mtFSCR for the 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 <npiggin@gmail.com>
> ---
>  arch/powerpc/kernel/exceptions-64s.S |  4 ++++
>  arch/powerpc/kernel/setup_64.c       | 28 ++++++++++++++++++++++++++++
>  arch/powerpc/kvm/Kconfig             |  9 +++++++++
>  arch/powerpc/kvm/book3s_pr.c         | 20 ++++++++++++++------
>  4 files changed, 55 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..7f7da641e551 100644
> --- a/arch/powerpc/kernel/setup_64.c
> +++ b/arch/powerpc/kernel/setup_64.c
> @@ -197,6 +197,34 @@ 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
> +		 *   while a PR guest is running.
> +		 *
> +		 * - SCV system call interrupt vectors are only implemented for
> +		 *   AIL mode interrupts.
> +		 *
> +		 * - On pseries, AIL mode can only be enabled and disabled
> +		 *   system-wide so when a PR VM is created on a pseries host,
> +		 *   all CPUs of the host are set to AIL=0 mode.
> +		 *
> +		 * - Therefore host CPUs must not execute scv while a PR VM
> +		 *   exists.
> +		 *
> +		 * - SCV support can not be disabled dynamically because the
> +		 *   feature is advertised to host userspace. Disabling the
> +		 *   facility and emulating it would be possible but is not
> +		 *   implemented.
> +		 *
> +		 * - So SCV support is blanket diabled if PR KVM could possibly

disabled

Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>