linux-kernel - Re: [Xen-devel] [PATCH] xen/hvc: If we use xen_raw

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Message-ID: <1377155718.31937.79.camel@hastur.hellion.org.uk>
Date:	Thu, 22 Aug 2013 08:15:18 +0100
From:	Ian Campbell <ian.campbell@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/hvc: If we use xen_raw_printk let it
 also work on HVM guests.

On Wed, 2013-08-21 at 06:44 -0400, Konrad Rzeszutek Wilk wrote:
> Ian Campbell <ian.campbell@...rix.com> wrote:
> >On Tue, 2013-08-20 at 15:35 -0400, Konrad Rzeszutek Wilk wrote:
> >>  void xen_raw_console_write(const char *str)
> >>  {
> >> -	dom0_write_console(0, str, strlen(str));
> >> +	ssize_t len = strlen(str);
> >> +	int rc = 0;
> >> +
> >> +	if (xen_domain()) {
> >> +		rc = dom0_write_console(0, str, len);
> >> +		if (rc != len && xen_hvm_domain()) /* -ENOSYS */
> >
> >If you want to catch ENOSYS then I suggest doing so explicitly, rather
> >that relying on len != -ENOSYS.
> >
> >> +			goto outb_print;
> >
> >How about reversing this into
> >	if (rc == len) return;
> >	if (rc != -ENOSYS) panic(...) /* yes, this won't get far... *.
> >
> >Then fall through to the following block as a plain if not an else if.
> >Maybe with a xen_hvm_domain && added.
> >
> >That avoids the yucky goto I think.
> >
> >> +	} else if (xen_cpuid_base()) {
> >> +		int i;
> >> +outb_print:
> >> +		for (i = 0; i < len; i++)
> >> +			outb(str[i], 0xe9);
> >> +	}
> >>  }
> >>  
> >>  void xen_raw_printk(const char *fmt, ...)
> 
> B/c the function  dom0_write_console never returns negative values.
> Instead it converts it to a zero return value. Perhaps it can return
> negative values -  will have to look. 

At least the virtio_console version of the hv_ops.put_chars hook can
return -errno. Perhaps this has changed since it was implemented.

Ian.

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