linux-kernel - Re: [Xen-devel] [PATCH 1/2] hvc

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Date:	Thu, 25 Feb 2016 12:00:30 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<xen-devel@...ts.xensource.com>, <linux-kernel@...r.kernel.org>,
	<david.vrabel@...rix.com>
Subject: Re: [Xen-devel] [PATCH 1/2] hvc_xen: add earlycon support

On Wed, 24 Feb 2016, Andrew Cooper wrote:
> On 24/02/16 17:18, Konrad Rzeszutek Wilk wrote:
> >> I could do the same here by dropping the if (!xen_pv_domain()) check
> >> above, but then if somebody specifies earlyprintk=xenboot on a non-Xen
> >> environment, I expect Linux would crash.
> > Nah, you made it "Work" with:
> > commit eb5ef07151ba3c3cb4bcef0c8f146ff1115eaa55
> > Author: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > Date:   Fri Jan 27 18:31:36 2012 +0000
> >
> >     hvc_xen: support PV on HVM consoles
> >
> > But this patch:
> >
> > commit 04b772d2b819f0dda2163e3193fa7cd447a6245c
> >
> > xen/hvc: If we use xen_raw_printk let it also work on HVM guests.
> >     
> >     The xen_raw_printk works great for debugging purposes. We use
> >     for PV guests and we can also use it for HVM guests.
> >     
> >     However, for HVM guests we have a fallback of using the 0xe9
> >     port in case the hypervisor does not support an HVM guest of
> >     using the console_io hypercall. As such lets use 0xe9 during
> >     early bootup, and once the hyper-page is setup and if the
> >     console_io hypercall is supported - use that. Otherwise we
> >     will fallback to using the 0xe9 after early bootup.
> >     
> >     We also alter the return value for dom0_write_console to return
> >     an error value instead of zero. The HVC API has been supporting
> >     returning error values for quite some time.
> >     
> >     P.S.
> >     To use (and to see the output in the Xen ring buffer) one has to build
> >     the hypervisor with 'debug=y'.
> >
> > Should make it possible for HVM guests to actually work with HVM x86 guests
> > if tweaked.
> 
> /me looks
> 
> +outb_print:
> +               for (i = 0; i < len; i++)
> +                       outb(str[i], 0xe9);
> +#endif
> 
> You already have the length to hand.  Use outsb instead, for
> substantially fewer vmexits.

I'll take the opportunity to make that change as well