Re: [Xen-devel] [PATCH 05/10] xen/arm: vpl011: Allocate a new PFN in the toolstack for the virtual console

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 13 Apr 2017, Wei Liu wrote:
> On Thu, Apr 13, 2017 at 02:07:54PM +0530, Bhupinder Thakur wrote:
> > Hi Wei,
> > 
> > 
> > >>  /* --- pluggable kernel loader ------------------------------------- */
> > >> diff --git a/tools/libxc/xc_dom_arm.c b/tools/libxc/xc_dom_arm.c
> > >> index a7e839e..157381e 100644
> > >> --- a/tools/libxc/xc_dom_arm.c
> > >> +++ b/tools/libxc/xc_dom_arm.c
> > >> @@ -26,10 +26,11 @@
> > >>  #include "xg_private.h"
> > >>  #include "xc_dom.h"
> > >>
> > >> -#define NR_MAGIC_PAGES 3
> > >> +#define NR_MAGIC_PAGES 4
> > >>  #define CONSOLE_PFN_OFFSET 0
> > >>  #define XENSTORE_PFN_OFFSET 1
> > >>  #define MEMACCESS_PFN_OFFSET 2
> > >> +#define VCONSOLE_PFN_OFFSET 3
> > >>
> > >>  #define LPAE_SHIFT 9
> > >>
> > >> @@ -85,6 +86,7 @@ static int alloc_magic_pages(struct xc_dom_image *dom)
> > >>
> > >>      dom->console_pfn = base + CONSOLE_PFN_OFFSET;
> > >>      dom->xenstore_pfn = base + XENSTORE_PFN_OFFSET;
> > >> +    dom->vconsole_pfn = base + VCONSOLE_PFN_OFFSET;
> > >>
> > >>      xc_clear_domain_page(dom->xch, dom->guest_domid, dom->console_pfn);
> > >>      xc_clear_domain_page(dom->xch, dom->guest_domid, dom->xenstore_pfn);
> > >> @@ -95,6 +97,9 @@ static int alloc_magic_pages(struct xc_dom_image *dom)
> > >>              dom->xenstore_pfn);
> > >>      xc_hvm_param_set(dom->xch, dom->guest_domid, 
> > >> HVM_PARAM_MONITOR_RING_PFN,
> > >>              base + MEMACCESS_PFN_OFFSET);
> > >> +    xc_hvm_param_set(dom->xch, dom->guest_domid, HVM_PARAM_VCONSOLE_PFN,
> > >> +                     base + VCONSOLE_PFN_OFFSET);
> > >> +
> > >
> > > Here is something I don't quite understand (sorry I haven't been
> > > following the conversation closely): if pl011 is emulated, why would the
> > > guest need to know its PFN?
> > 
> > This PFN is used by Xen to setup a ring-buffer between xenconsole and 
> > itself.
> > Xen reads/writes data from/to this ring buffer when it gets a mmio
> > read/write request from the guest.
> > 
> 
> What I was getting at was "does the *guest* need to know the PFN"?  The
> hypervisor and xenconsole daemon / client aren't the guest. Does the
> guest need to know the exact PFN in order to setup MMIO?
> 
> Ultimately this is going to be decided by ARM maintainers. I'm just
> curious about why it is done like this.

hvm_params are commonly used to pass parameters from Xen or from libxl
to guests. In this case, they are used to pass parameters from the
toolstack to Xen.

The guest does not need to know the pfn, and in fact it cannot: Xen
refuses to return the value of HVM_PARAM_VCONSOLE_PFN to guests (see
patch #2).

Honestly, I don't particularly care about how the pfn is passed from
libxc to Xen. hvm_param is an option, or we could introduce a new
domctl.

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