Re: [PATCH 03/14] KVM: arm64: Add support for creating and checking a logging buffer inside hyp/nVHE

[Andrew Scull]

On Mon, Sep 14, 2020 at 05:27:39PM +0000, George-Aurelian Popescu wrote:
> From: George Popescu <georgepope@xxxxxxxxxx>
> 
> Share a buffer between the kernel and the hyp/nVHE code by using the
> macros from kvm_debug_buffer.h.
> 
> The buffer is composed of a writing index and a statically allocated
> array. The writing index counts how many elements have been written inside
> the buffer and should be set to zero whenever the code goes back to
> EL2 with the clear_kvm_debug_buffer macro.
> 
> To avoid consistency problems the buffer is defined per_cpu and is designed
> to be read-only from the kernel perspective.
> 
> Check if there is any logging data from hyp/nVHE code.
> 
> Every time when the state returns back to the kernel after an hvc call,
> the __kvm_arm_check_debug_buffer macro checks if there is any data inside
> one of the predefined buffers.
> 
> Signed-off-by: George Popescu <georgepope@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_debug_buffer.h | 34 +++++++++++++++++++++++
>  arch/arm64/include/asm/kvm_host.h         |  6 ++++
>  arch/arm64/kvm/hyp/hyp-entry.S            |  2 +-
>  3 files changed, 41 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/kvm_debug_buffer.h
> 
> diff --git a/arch/arm64/include/asm/kvm_debug_buffer.h b/arch/arm64/include/asm/kvm_debug_buffer.h
> new file mode 100644
> index 000000000000..30c9b0b1a7bf
> --- /dev/null
> +++ b/arch/arm64/include/asm/kvm_debug_buffer.h
> @@ -0,0 +1,34 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright 2020 Google LLC
> + * Author: George Popescu <georgepope@xxxxxxxxxx>
> + */
> +#ifndef __ASSEMBLY__
> +
> +#include <linux/percpu-defs.h>
> +#include <asm/kvm_asm.h>
> +
> +#ifdef __KVM_NVHE_HYPERVISOR__
> +#define DEFINE_KVM_DEBUG_BUFFER(type_name, buff_name, size)             \
> +	DEFINE_PER_CPU(type_name, buff_name)[(size)];	                \
> +	DEFINE_PER_CPU(unsigned long, buff_name##_wr_ind) = 0
> +
> +#define DECLARE_KVM_DEBUG_BUFFER(type_name, buff_name, size)            \
> +	DECLARE_PER_CPU(type_name, buff_name)[(size)];                  \
> +	DECLARE_PER_CPU(unsigned long, buff_name##_wr_ind)
> +
> +#else
> +
> +#define DECLARE_KVM_DEBUG_BUFFER(type_name, buff_name, size)            \
> +	DECLARE_PER_CPU(type_name, kvm_nvhe_sym(buff_name))[(size)];    \
> +	DECLARE_PER_CPU(unsigned long, kvm_nvhe_sym(buff_name##_wr_ind))
> +#endif //__KVM_NVHE_HYPERVISOR__

nit: comment style, here and below

> +
> +#else
> +
> +.macro clear_kvm_debug_buffer sym tmp1, tmp2, tmp3
> +	mov \tmp1, 0
> +	hyp_str_this_cpu \sym, \tmp1, \tmp2, \tmp3
> +.endm

Can you can use xzr (zero register) directly rather than moving the
constant 0 into a temporary?

	hyp_str_this_cpu \sym, xzr, \tmp1, \tmp2

> +
> +#endif // __ASSEMBLY__
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 905c2b87e05a..adc8957e9321 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -494,6 +494,10 @@ u64 __kvm_call_hyp(void *hypfn, ...);
>  		__kvm_call_hyp(kvm_ksym_ref_nvhe(f), ##__VA_ARGS__);	\
>  	})
>  
> +#define __kvm_arm_check_debug_buffer()					\
> +{									\
> +}
> +
>  /*
>   * The couple of isb() below are there to guarantee the same behaviour
>   * on VHE as on !VHE, where the eret to EL1 acts as a context
> @@ -506,6 +510,7 @@ u64 __kvm_call_hyp(void *hypfn, ...);
>  			isb();						\
>  		} else {						\
>  			kvm_call_hyp_nvhe(f, ##__VA_ARGS__);		\
> +			__kvm_arm_check_debug_buffer();			\
>  		}							\
>  	} while(0)
>  
> @@ -518,6 +523,7 @@ u64 __kvm_call_hyp(void *hypfn, ...);
>  			isb();						\
>  		} else {						\
>  			ret = kvm_call_hyp_nvhe_ret(f, ##__VA_ARGS__);	\
> +			__kvm_arm_check_debug_buffer();			\

As Will was pointing out earlier, does the checking need to have
preemption disabled in case there is another call into hyp that corrupts
the buffer while it is being checked?

>  		}							\
>  									\
>  		ret;							\
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> index 46b4dab933d0..8df0082b9ccf 100644
> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -68,7 +68,7 @@ el1_sync:				// Guest trapped into EL2
>  	cbnz	x1, el1_hvc_guest	// called HVC
>  
>  	/* Here, we're pretty sure the host called HVC. */
> -	ldp	x0, x1, [sp], #16
> +	ldp	x0, x1,	[sp], #16

Is this a whitespace change? Maybe drop from this patch if it isn't
related.

>  
>  	/* Check for a stub HVC call */
>  	cmp	x0, #HVC_STUB_HCALL_NR
> -- 
> 2.28.0.618.gf4bc123cb7-goog
>