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From: Eric Blake <>
To: Farhan Ali <>,
	Cornelia Huck <>,
	Christian Borntraeger <>,
	Thomas Huth <>
Subject: Re: [Qemu-devel] [PATCH] s390-ccw: force diag 308 subcode to unsigned long
Date: Thu, 3 May 2018 10:48:10 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 05/03/2018 10:25 AM, Farhan Ali wrote:
> On 05/02/2018 08:52 AM, Cornelia Huck wrote:
>> We currently pass an integer as the subcode parameter. However,
>> the upper bits of the register containing the subcode need to
>> be 0, which is not guaranteed unless we explicitly specify the
>> subcode to be an unsigned long value.
>> Fixes: d046c51dad3 ("pc-bios/s390-ccw: Get device address via diag 
>> 308/6")
>> Signed-off-by: Cornelia Huck<>
> Sorry for my ignorance, but is there a C standard that says upper bits 
> of an int is not guaranteed to be 0?

We're outside the bounds of the C standard because of the use of asm(). 
The problem here is that the compiler assigning a 32-bit int into a 
64-bit register uses the shortest sequence possible (leaving the upper 
64 bits garbage), because the compiler assumes you correctly wrote the 
assembly to only use 32-bit operations on that register (which don't 
care about the upper bits).  By using an unsigned long (a 64-bit value), 
the compiler instead emits assembly to write the full 64-bit register 
value, rather than leaving the upper bits as garbage; and this matters 
because we are subsequently using all 64 bits of the register in a later 
operation.  We could also use a signed long, even long long, or written 
it as: (store ? 6ULL : 5ULL) instead of using a temporary variable.  The 
crux of the fix is that you have to tell asm() that you want a 64-bit 
value written (the unpatched (store ? 6 : 5) is only a 32-bit value), 
and not whether that value is signed or unsigned (since the 
representation of both 6 and 5 are the same regardless of whether the 
type being written into the register is signed or not).

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization: |

  parent reply	other threads:[~2018-05-03 15:48 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-02 12:52 [Qemu-devel] [PATCH] s390-ccw: force diag 308 subcode to unsigned long Cornelia Huck
2018-05-02 12:56 ` Christian Borntraeger
2018-05-02 13:32 ` Thomas Huth
2018-05-03 15:20 ` [Qemu-devel] [qemu-s390x] " David Hildenbrand
2018-05-03 15:25 ` [Qemu-devel] " Farhan Ali
2018-05-03 15:44   ` Cornelia Huck
2018-05-03 16:05     ` Farhan Ali
2018-05-03 15:48   ` Eric Blake [this message]
2018-05-03 16:03     ` Farhan Ali

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