Subject: Re: [PATCH v2] KVM: PPC: Exit guest upon fatal machine check exception

From: Thomas Huth <[email-masked]>
Date: Wed, 16 Dec 2015 10:40:28 +0100

On 16/12/15 06:56, Aravinda Prasad wrote:
> This patch modifies KVM to cause a guest exit with
> KVM_EXIT_NMI instead of immediately delivering a 0x200
> interrupt to guest upon machine check exception in
> guest address. Exiting the guest enables QEMU to build
> error log and deliver machine check exception to guest
> OS (either via guest OS registered machine check
> handler or via 0x200 guest OS interrupt vector).
> 
> This approach simplifies the delivering of machine
> check exception to guest OS compared to the earlier approach
> of KVM directly invoking 0x200 guest interrupt vector.
> In the earlier approach QEMU patched the 0x200 interrupt
> vector during boot. The patched code at 0x200 issued a
> private hcall to pass the control to QEMU to build the
> error log.
> 
> This design/approach is based on the feedback for the
> QEMU patches to handle machine check exception. Details
> of earlier approach of handling machine check exception
> in QEMU and related discussions can be found at:
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2014-11/msg00813.html
> 
> This patch also introduces a new KVM capability to
> control how KVM behaves on machine check exception.
> Without this capability, KVM redirects machine check
> exceptions to guest's 0x200 vector if the address in
> error belongs to guest. With this capability KVM
> causes a guest exit with NMI exit reason.
> 
> This is required to avoid problems if a new kernel/KVM
> is used with an old QEMU for guests that don't issue
> "ibm,nmi-register". As old QEMU does not understand the
> NMI exit type, it treats it as a fatal error. However,
> the guest could have handled the machine check error
> if the exception was delivered to guest's 0x200 interrupt
> vector instead of NMI exit in case of old QEMU.
> 
> Change Log v2:
>   - Added KVM capability
> 
> Signed-off-by: Aravinda Prasad <[email-masked]>
> ---
>  arch/powerpc/include/asm/kvm_host.h     |    1 +
>  arch/powerpc/kernel/asm-offsets.c       |    1 +
>  arch/powerpc/kvm/book3s_hv.c            |   12 +++-------
>  arch/powerpc/kvm/book3s_hv_rmhandlers.S |   37 +++++++++++++++----------------
>  arch/powerpc/kvm/powerpc.c              |    7 ++++++
>  include/uapi/linux/kvm.h                |    1 +
>  6 files changed, 31 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> index 827a38d..8a652ba 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -243,6 +243,7 @@ struct kvm_arch {
>  	int hpt_cma_alloc;
>  	struct dentry *debugfs_dir;
>  	struct dentry *htab_dentry;
> +	u8 fwnmi_enabled;

Here you declare the variable as 8-bits ...

>  #endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */
>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
>  	struct mutex hpt_mutex;
> diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
> index 221d584..6a4e81a 100644
> --- a/arch/powerpc/kernel/asm-offsets.c
> +++ b/arch/powerpc/kernel/asm-offsets.c
> @@ -506,6 +506,7 @@ int main(void)
>  	DEFINE(KVM_ENABLED_HCALLS, offsetof(struct kvm, arch.enabled_hcalls));
>  	DEFINE(KVM_LPCR, offsetof(struct kvm, arch.lpcr));
>  	DEFINE(KVM_VRMA_SLB_V, offsetof(struct kvm, arch.vrma_slb_v));
> +	DEFINE(KVM_FWNMI, offsetof(struct kvm, arch.fwnmi_enabled));

... then define an asm-offset for it ...

>  	DEFINE(VCPU_DSISR, offsetof(struct kvm_vcpu, arch.shregs.dsisr));
>  	DEFINE(VCPU_DAR, offsetof(struct kvm_vcpu, arch.shregs.dar));
>  	DEFINE(VCPU_VPA, offsetof(struct kvm_vcpu, arch.vpa.pinned_addr));
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index b98889e..f43c124 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
[...]
> @@ -2381,24 +2377,27 @@ machine_check_realmode:
>  	ld	r9, HSTATE_KVM_VCPU(r13)
>  	li	r12, BOOK3S_INTERRUPT_MACHINE_CHECK
>  	/*
> -	 * Deliver unhandled/fatal (e.g. UE) MCE errors to guest through
> -	 * machine check interrupt (set HSRR0 to 0x200). And for handled
> -	 * errors (no-fatal), just go back to guest execution with current
> -	 * HSRR0 instead of exiting guest. This new approach will inject
> -	 * machine check to guest for fatal error causing guest to crash.
> -	 *
> -	 * The old code used to return to host for unhandled errors which
> -	 * was causing guest to hang with soft lockups inside guest and
> -	 * makes it difficult to recover guest instance.
> +	 * Deliver unhandled/fatal (e.g. UE) MCE errors to guest
> +	 * by exiting the guest with KVM_EXIT_NMI exit reason (exit
> +	 * reason set later based on trap). For handled errors
> +	 * (no-fatal), go back to guest execution with current HSRR0
> +	 * instead of exiting the guest. This approach will cause
> +	 * the guest to exit in case of fatal machine check error.
>  	 */
> -	ld	r10, VCPU_PC(r9)
> -	ld	r11, VCPU_MSR(r9)
> -	bne	2f	/* Continue guest execution. */
> -	/* If not, deliver a machine check.  SRR0/1 are already set */
> -	li	r10, BOOK3S_INTERRUPT_MACHINE_CHECK
> +	bne	2f	/* Continue guest execution? */
> +	/* If not, check if guest is capable of handling NMI exit */
> +	ld	r3, VCPU_KVM(r9)
> +	ld	r3, KVM_FWNMI(r3)
> +	cmpdi	r3, 1		/* FWNMI capable? */

... and here you're accessing the 8-bit variable with "ld" and "cmpdi"!
Is this really working as expected? Or did I miss something? Did you
check your code on both, little and big endian hosts?

 Thomas