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Subject: Re: [Xen-devel] [PATCH] x86/xen: fix pv boot
From:   Andrew Cooper <andrew.cooper3@citrix.com>
To:     Juergen Gross <jgross@suse.com>, <linux-kernel@vger.kernel.org>,
        <xen-devel@lists.xenproject.org>, <x86@kernel.org>
CC:     <sstabellini@kernel.org>, <mingo@redhat.com>, <bp@alien8.de>,
        <hpa@zytor.com>, <boris.ostrovsky@oracle.com>, <tglx@linutronix.de>
References: <20181107113017.30090-1-jgross@suse.com>
 <f95cf3fb-fb49-8211-0c54-f6c7382ac455@citrix.com>
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Message-ID: <6eb96a45-393c-4e06-ee19-549fdbe9e316@citrix.com>
Date:   Wed, 7 Nov 2018 12:02:16 +0000
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On 07/11/18 11:55, Andrew Cooper wrote:
> On 07/11/18 11:30, Juergen Gross wrote:
>> +	return ret;
>>  }
>>  
>>  static inline int xen_safe_read_ulong(unsigned long *addr, unsigned long *val)
>>  {
>> -	return __get_user(*val, (unsigned long __user *)addr);
>> +	int ret = 0;
>> +	unsigned long rval;
>> +
>> +	asm volatile("1: mov"__i" %2,%"__r"1\n"
>> +		     "2:\n"
>> +		     ".section .fixup,\"ax\"\n"
>> +		     "3: mov %3,%0\n"
>> +		     "   jmp 2b\n"
>> +		     ".previous\n"
>> +		     _ASM_EXTABLE(1b, 3b)
>> +		     : "=r" (ret), "=r" (rval)
>> +		     : "m" (*addr), "i" (-1), "0" (ret));
>> +	*val = rval;
>> +	return ret;
> This also uses rval uninitialised if a fault occurs.
>
> Overall, how about:
>
> static inline int xen_safe_write_ulong(unsigned long *addr, unsigned
> long val)
> {
>     int ret = 0;
>
>     asm volatile("1: mov %[val], %[ptr]\n"
>                  "2:\n"
>                  ".section .fixup, \"ax\"\n"
>                  "3: mov %[err], %[ret]\n"

Actually, if you want a shorter fixup path, `sub $1, %[ret]` would also
do, and drop the [err] constraint.

~Andrew