linux-kernel - Re: [PATCH v3 1/2] s390/setup: diag318: remove bit check and refactor struct

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Message-ID: <599addd3-a4f8-d4e7-5898-dc45f52cd7a6@redhat.com>
Date:   Wed, 3 Apr 2019 14:03:21 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Collin Walling <walling@...ux.ibm.com>, pbonzini@...hat.com,
        cohuck@...hat.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Cc:     schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        borntraeger@...ibm.com, frankja@...ux.ibm.com
Subject: Re: [PATCH v3 1/2] s390/setup: diag318: remove bit check and refactor
 struct

On 02.04.19 19:46, Collin Walling wrote:
> Execution of DIAGNOSE 0x318 is fenced by checking an SCLP bit
> for the availability of hardware support for the instruction.
> 
> In order to support this instruction for a KVM/QEMU guest, we
> would need to provide modifications to the SCLP Read SCP Info
> data, which will in turn reduce the maximum number of CPUs that
> may be provided to the guest. This issue introduces compatability
> and legacy concerns.
> 
> Let's circumvent this issue by removing the bit check and blindly
> executing the instruction. An exception table rule is in place to
> catch the case where hardware does not support this instruction.
> 
> While we're at it, let's condense the version code fields in the
> diag318_info struct until we can determine how it will be used.
> 
> This modifies commit 4ad78b8651aacf26b3ab6d1e784952eb70469c43
> 
> Signed-off-by: Collin Walling <walling@...ux.ibm.com>
> ---
>  arch/s390/include/asm/diag.h |  6 ++----
>  arch/s390/kernel/setup.c     | 12 ++++++------
>  2 files changed, 8 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/s390/include/asm/diag.h b/arch/s390/include/asm/diag.h
> index 19562be22b7e..215516284175 100644
> --- a/arch/s390/include/asm/diag.h
> +++ b/arch/s390/include/asm/diag.h
> @@ -298,10 +298,8 @@ struct diag26c_mac_resp {
>  union diag318_info {
>  	unsigned long val;
>  	struct {
> -		unsigned int cpnc : 8;
> -		unsigned int cpvc_linux : 24;
> -		unsigned char cpvc_distro[3];
> -		unsigned char zero;
> +		unsigned long cpnc : 8;
> +		unsigned long cpvc : 56;
>  	};
>  };
>  
> diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
> index 2c642af526ce..fe70201f8b5d 100644
> --- a/arch/s390/kernel/setup.c
> +++ b/arch/s390/kernel/setup.c
> @@ -1011,15 +1011,15 @@ static void __init setup_control_program_code(void)
>  {
>  	union diag318_info diag318_info = {
>  		.cpnc = CPNC_LINUX,
> -		.cpvc_linux = 0,
> -		.cpvc_distro = {0},
> +		.cpvc = 0,
>  	};
>  
> -	if (!sclp.has_diag318)
> -		return;
> -
>  	diag_stat_inc(DIAG_STAT_X318);
> -	asm volatile("diag %0,0,0x318\n" : : "d" (diag318_info.val));
> +	asm volatile(
> +		"	diag	%0,0,0x318\n"
> +		"0:	nopr	%%r7\n"
> +		EX_TABLE(0b,0b)
> +		: : "d" (diag318_info.val));
>  }
>  
>  /*
> 

That smells like a nasty hack to not expose new features in QEMU and
deal with the issue of handling CPU limits. No, I don't like this.

Fix QEMU, not the kernel.

-- 

Thanks,

David / dhildenb