Re: [PATCH] xen/x86: Improve hypercall page writing

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	SuraveeSuthikulpanit <suravee.suthikulpanit@amd.com>,
	Xen-devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH] xen/x86: Improve hypercall page writing
Date: Tue, 20 Dec 2016 12:21:46 +0000	[thread overview]
Message-ID: <6db9b2f2-4c13-48f2-6b3c-67a435c32131@citrix.com> (raw)
In-Reply-To: <5858F05A020000780012AC80@prv-mh.provo.novell.com>

On 20/12/2016 07:48, Jan Beulich wrote:
>
>> @@ -398,10 +400,11 @@ static void hypercall_page_initialise_ring1_kernel(void *hypercall_page)
>>       * calling it.
>>       */
>>      p = (char *)(hypercall_page + (__HYPERVISOR_iret * 32));
>> -    *(u8  *)(p+ 0) = 0x50;    /* push %eax */
>> -    *(u8  *)(p+ 1) = 0xb8;    /* mov  $__HYPERVISOR_iret,%eax */
>> -    *(u32 *)(p+ 2) = __HYPERVISOR_iret;
>> -    *(u16 *)(p+ 6) = (HYPERCALL_VECTOR << 8) | 0xcd; /* int  $xx */
>> +    memcpy(p,
>> +           "\x50"                 /* push %eax */
>> +           "\xb8\x17\x00\x00\x00" /* mov  $__HYPERVISOR_iret, %eax */
>> +           "\xcd\x82",            /* int  $HYPERCALL_VECTOR */
> here as a good idea.

Well, they are fixed in the ABI.  It is not as if we could ever change them.

>  If you used a static const uint8_t[] instead of
> a string literal (which even includes a pointless nul terminator), all of
> this could be avoided afaict.

I can see how that would work for the `int $0x82` case, but how do you
propose fitting 4 bytes of __HYPERVISOR_iret into an initialiser for a
uint8_t array?

~Andrew

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