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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Apr 18, 2017 at 05:36:41PM -0700, Stefano Stabellini wrote:
> 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).
> 

Ah, I missed the new restriction in #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.

No opinion from me either. I think using HVM params is fine.

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