[PATCH v2 0/3] xenconsole: Add connection flag

[PATCH v2 0/3] xenconsole: Add connection flag

[Jason Andryuk]

Add a connection flag to the console interface page so a domain can tell
if it is connected or not.  This became a series in v2 to add flag
setting to libxenguest.

Jason Andryuk (3):
  xenconsole: Add connection flag
  libs/guest: Set console page to disconnected
  libs/guest: Set console as disconnected on resume

 tools/console/daemon/io.c                |  4 +++
 tools/include/xenguest.h                 |  4 +++
 tools/libs/guest/xg_dom_arm.c            |  2 +-
 tools/libs/guest/xg_dom_boot.c           | 36 ++++++++++++++++++++++++
 tools/libs/guest/xg_dom_x86.c            |  6 ++--
 tools/libs/guest/xg_sr_restore_x86_hvm.c |  2 +-
 tools/libs/guest/xg_sr_restore_x86_pv.c  |  1 +
 xen/include/public/io/console.h          | 13 +++++++++
 8 files changed, 63 insertions(+), 5 deletions(-)

-- 
2.50.1

[PATCH v2 1/3] xenconsole: Add connection flag

[Jason Andryuk]

With hyperlaunch, a domU can start before its console ring is connected
by xenconsoled.  With nothing emptying the ring, it can quickly fill
during boot.  In domU_write_console(), __write_console() returns 0 when
the ring is full.  This loops spins until xenconsoled starts emptying
the ring:

        while (len) {
                ssize_t sent = __write_console(cons, data, len);

                if (sent < 0)
                        return sent;

                data += sent;
                len -= sent;

                if (unlikely(len))
                        HYPERVISOR_sched_op(SCHEDOP_yield, NULL);
        }

The goal of this patch is to add a way for the frontend to know when a
console is connected.  This patch adds a new flag to the end of the
console ring structure.  It is used for the backend to indicate that it
has connected and started servicing the page.

The two values are
XENCONSOLE_DISCONNECTED 1
XENCONSOLE_CONNECTED    0

XENCONSOLE_DISCONNECTED indicates to the guest that ring is
disconnected, so it will not be serviced.  The guest can avoid writing
into it in that case.  A domU can use console hypercalls and only
transition to the ring when it is connected and won't fill and block.

Once the backend (xenconsoled) maps and starts servicing the
console, the flag will be set to XENCONSOLE_CONNECTED (0) to indicate
the backend state to the frontend.

The connected value as 0 will be match the default of a zero-ed console
page.  Hyperlaunch can set the flag to XENCONSOLE_DISCONNECTED and let
xenconsoled set to XENCONSOLE_CONNECTED.

Old domU hvc_xen drivers won't check the flag.
New domU hvc_xen running on a new xen/xenconsoled will work properly.
New domU hvc_xen on old xen/xenconsoled should only see a 0 for the flag
and behave as if connected.

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
---
v2:
Rename "connection"
Use uint8_t

v1:
Remove evtchn notify call
Set connected later when there is no error

RFC v3:
Flip flag values so 0 is connected.

The other option would be to add:
uint32_t features
uint32_t connected

New domUs would check features for a magic value and/or flag to know
they can rely on connected transitioning.

I think making XENCONSOLE_CONNECTED == 0 side steps the need for
an additional features field.  As long as assuming zero-ed memory is
acceptable.  However, this only matters for a hyperlaunched guest -
xenconsoled will normally readily connect the console and set the value.

This assumes that existing frontends are not using the flag space for
some other use.
---
 tools/console/daemon/io.c       |  4 ++++
 xen/include/public/io/console.h | 13 +++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/tools/console/daemon/io.c b/tools/console/daemon/io.c
index bb739bdb8c..43d4973c24 100644
--- a/tools/console/daemon/io.c
+++ b/tools/console/daemon/io.c
@@ -781,6 +781,10 @@ static int console_create_ring(struct console *con)
 		con->log_fd = create_console_log(con);
 
  out:
+	/* Mark the console connected. */
+	if (!err && con->interface)
+		con->interface->connection = XENCONSOLE_CONNECTED;
+
 	return err;
 }
 
diff --git a/xen/include/public/io/console.h b/xen/include/public/io/console.h
index 4509b4b689..b9ebfaff3f 100644
--- a/xen/include/public/io/console.h
+++ b/xen/include/public/io/console.h
@@ -19,6 +19,19 @@ struct xencons_interface {
     char out[2048];
     XENCONS_RING_IDX in_cons, in_prod;
     XENCONS_RING_IDX out_cons, out_prod;
+/*
+ * Flag values signaling from backend to frontend whether the console is
+ * connected.  i.e. Whether it will be serviced and emptied.
+ *
+ * The flag starts as disconnected.
+ */
+#define XENCONSOLE_DISCONNECTED 1
+/*
+ * The flag is set to connected when the backend connects and the console
+ * will be serviced.
+ */
+#define XENCONSOLE_CONNECTED    0
+    uint8_t connection;
 };
 
 #ifdef XEN_WANT_FLEX_CONSOLE_RING
-- 
2.50.1

[PATCH v2 2/3] libs/guest: Set console page to disconnected

[Jason Andryuk]

Initialize xencons_interface's connection field to
XENCONSOLE_DISCONNECTED.  xenconsoled will mark the page as connected
when it establishes the connection.

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
---
xc_dom_console_init() seems closer to the functionality of the gnttab
functions, so I put it in xg_dom_boot.c.  It can't take a struct
xc_dom_image pointer when used with save/restore.

v2:
New
---
 tools/include/xenguest.h       |  2 ++
 tools/libs/guest/xg_dom_arm.c  |  2 +-
 tools/libs/guest/xg_dom_boot.c | 20 ++++++++++++++++++++
 tools/libs/guest/xg_dom_x86.c  |  6 +++---
 4 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/tools/include/xenguest.h b/tools/include/xenguest.h
index e01f494b77..1d5a6d3509 100644
--- a/tools/include/xenguest.h
+++ b/tools/include/xenguest.h
@@ -335,6 +335,8 @@ void *xc_dom_boot_domU_map(struct xc_dom_image *dom, xen_pfn_t pfn,
                            xen_pfn_t count);
 int xc_dom_boot_image(struct xc_dom_image *dom);
 int xc_dom_compat_check(struct xc_dom_image *dom);
+int xc_dom_console_init(xc_interface *xch, uint32_t guest_domid,
+                        xen_pfn_t console_gfn);
 int xc_dom_gnttab_init(struct xc_dom_image *dom);
 int xc_dom_gnttab_seed(xc_interface *xch, uint32_t guest_domid,
                        bool is_hvm,
diff --git a/tools/libs/guest/xg_dom_arm.c b/tools/libs/guest/xg_dom_arm.c
index 2fd8ee7ad4..c8d0918506 100644
--- a/tools/libs/guest/xg_dom_arm.c
+++ b/tools/libs/guest/xg_dom_arm.c
@@ -70,7 +70,7 @@ static int alloc_magic_pages(struct xc_dom_image *dom)
     dom->xenstore_pfn = base + XENSTORE_PFN_OFFSET;
     dom->vuart_gfn = base + VUART_PFN_OFFSET;
 
-    xc_clear_domain_page(dom->xch, dom->guest_domid, dom->console_pfn);
+    xc_dom_console_init(dom->xch, dom->guest_domid, dom->console_pfn);
     xc_clear_domain_page(dom->xch, dom->guest_domid, dom->xenstore_pfn);
     xc_clear_domain_page(dom->xch, dom->guest_domid, base + MEMACCESS_PFN_OFFSET);
     xc_clear_domain_page(dom->xch, dom->guest_domid, dom->vuart_gfn);
diff --git a/tools/libs/guest/xg_dom_boot.c b/tools/libs/guest/xg_dom_boot.c
index 5c7e12221d..b5f248e642 100644
--- a/tools/libs/guest/xg_dom_boot.c
+++ b/tools/libs/guest/xg_dom_boot.c
@@ -34,6 +34,7 @@
 #include "xg_core.h"
 #include <xen/hvm/params.h>
 #include <xen/grant_table.h>
+#include <xen/io/console.h>
 
 /* ------------------------------------------------------------------------ */
 
@@ -427,6 +428,25 @@ int xc_dom_gnttab_init(struct xc_dom_image *dom)
                               dom->console_domid, dom->xenstore_domid);
 }
 
+int xc_dom_console_init(xc_interface *xch,
+                        uint32_t domid,
+                        unsigned long dst_pfn)
+{
+    const size_t size = PAGE_SIZE;
+    struct xencons_interface *xencons = xc_map_foreign_range(
+        xch, domid, size, PROT_WRITE, dst_pfn);
+
+    if ( xencons == NULL )
+        return -1;
+
+    memset(xencons, 0, size);
+    xencons->connection = XENCONSOLE_DISCONNECTED;
+
+    munmap(xencons, size);
+    xc_domain_cacheflush(xch, domid, dst_pfn, 1);
+    return 0;
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/libs/guest/xg_dom_x86.c b/tools/libs/guest/xg_dom_x86.c
index cba01384ae..a82b481a12 100644
--- a/tools/libs/guest/xg_dom_x86.c
+++ b/tools/libs/guest/xg_dom_x86.c
@@ -587,8 +587,8 @@ static int alloc_magic_pages_pv(struct xc_dom_image *dom)
     dom->console_pfn = xc_dom_alloc_page(dom, "console");
     if ( dom->console_pfn == INVALID_PFN )
         return -1;
-    xc_clear_domain_page(dom->xch, dom->guest_domid,
-                         xc_dom_p2m(dom, dom->console_pfn));
+    xc_dom_console_init(dom->xch, dom->guest_domid,
+                        xc_dom_p2m(dom, dom->console_pfn));
 
     dom->alloc_bootstack = 1;
 
@@ -734,7 +734,7 @@ static int alloc_magic_pages_hvm(struct xc_dom_image *dom)
                      special_pfn(SPECIALPAGE_IDENT_PT) << PAGE_SHIFT);
 
     dom->console_pfn = special_pfn(SPECIALPAGE_CONSOLE);
-    xc_clear_domain_page(dom->xch, dom->guest_domid, dom->console_pfn);
+    xc_dom_console_init(dom->xch, dom->guest_domid, dom->console_pfn);
 
     dom->xenstore_pfn = special_pfn(SPECIALPAGE_XENSTORE);
     xc_clear_domain_page(dom->xch, dom->guest_domid, dom->xenstore_pfn);
-- 
2.50.1

[PATCH v2 3/3] libs/guest: Set console as disconnected on resume

[Jason Andryuk]

There is currently an asymmetry between HVM where the page is cleared
and PV where the contents are restored.

Add xc_dom_console_set_disconnected() to only set the connection flag
for PV guests.

xenconsoled is responsible for setting the console connected when it
attaches.

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
---
v2:
New
---
 tools/include/xenguest.h                 |  2 ++
 tools/libs/guest/xg_dom_boot.c           | 24 ++++++++++++++++++++----
 tools/libs/guest/xg_sr_restore_x86_hvm.c |  2 +-
 tools/libs/guest/xg_sr_restore_x86_pv.c  |  1 +
 4 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/tools/include/xenguest.h b/tools/include/xenguest.h
index 1d5a6d3509..c88958faa9 100644
--- a/tools/include/xenguest.h
+++ b/tools/include/xenguest.h
@@ -337,6 +337,8 @@ int xc_dom_boot_image(struct xc_dom_image *dom);
 int xc_dom_compat_check(struct xc_dom_image *dom);
 int xc_dom_console_init(xc_interface *xch, uint32_t guest_domid,
                         xen_pfn_t console_gfn);
+int xc_dom_console_set_disconnected(xc_interface *xch, uint32_t guest_domid,
+                                    xen_pfn_t console_gfn);
 int xc_dom_gnttab_init(struct xc_dom_image *dom);
 int xc_dom_gnttab_seed(xc_interface *xch, uint32_t guest_domid,
                        bool is_hvm,
diff --git a/tools/libs/guest/xg_dom_boot.c b/tools/libs/guest/xg_dom_boot.c
index b5f248e642..f51b6a78c8 100644
--- a/tools/libs/guest/xg_dom_boot.c
+++ b/tools/libs/guest/xg_dom_boot.c
@@ -428,9 +428,10 @@ int xc_dom_gnttab_init(struct xc_dom_image *dom)
                               dom->console_domid, dom->xenstore_domid);
 }
 
-int xc_dom_console_init(xc_interface *xch,
-                        uint32_t domid,
-                        unsigned long dst_pfn)
+static int dom_console_init(xc_interface *xch,
+                            uint32_t domid,
+                            unsigned long dst_pfn,
+                            bool clear)
 {
     const size_t size = PAGE_SIZE;
     struct xencons_interface *xencons = xc_map_foreign_range(
@@ -439,7 +440,8 @@ int xc_dom_console_init(xc_interface *xch,
     if ( xencons == NULL )
         return -1;
 
-    memset(xencons, 0, size);
+    if (clear)
+        memset(xencons, 0, size);
     xencons->connection = XENCONSOLE_DISCONNECTED;
 
     munmap(xencons, size);
@@ -447,6 +449,20 @@ int xc_dom_console_init(xc_interface *xch,
     return 0;
 }
 
+int xc_dom_console_init(xc_interface *xch,
+                        uint32_t domid,
+                        unsigned long dst_pfn)
+{
+    return dom_console_init(xch, domid, dst_pfn, true);
+}
+
+int xc_dom_console_set_disconnected(xc_interface *xch,
+                                    uint32_t domid,
+                                    unsigned long dst_pfn)
+{
+    return dom_console_init(xch, domid, dst_pfn, false);
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/libs/guest/xg_sr_restore_x86_hvm.c b/tools/libs/guest/xg_sr_restore_x86_hvm.c
index d6ea6f3012..a5d1511fde 100644
--- a/tools/libs/guest/xg_sr_restore_x86_hvm.c
+++ b/tools/libs/guest/xg_sr_restore_x86_hvm.c
@@ -62,7 +62,7 @@ static int handle_hvm_params(struct xc_sr_context *ctx,
         {
         case HVM_PARAM_CONSOLE_PFN:
             ctx->restore.console_gfn = entry->value;
-            xc_clear_domain_page(xch, ctx->domid, entry->value);
+            xc_dom_console_init(xch, ctx->domid, entry->value);
             break;
         case HVM_PARAM_STORE_PFN:
             ctx->restore.xenstore_gfn = entry->value;
diff --git a/tools/libs/guest/xg_sr_restore_x86_pv.c b/tools/libs/guest/xg_sr_restore_x86_pv.c
index 9cd6a88022..876748c11e 100644
--- a/tools/libs/guest/xg_sr_restore_x86_pv.c
+++ b/tools/libs/guest/xg_sr_restore_x86_pv.c
@@ -208,6 +208,7 @@ static int process_start_info(struct xc_sr_context *ctx,
         goto err;
     }
 
+    xc_dom_console_set_disconnected(xch, ctx->domid, mfn);
     ctx->restore.console_gfn = mfn;
     SET_FIELD(guest_start_info, console.domU.mfn, mfn, ctx->x86.pv.width);
     SET_FIELD(guest_start_info, console.domU.evtchn,
-- 
2.50.1

Re: [PATCH v2 0/3] xenconsole: Add connection flag

[Jürgen Groß]

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On 22.08.25 23:39, Jason Andryuk wrote:
> Add a connection flag to the console interface page so a domain can tell
> if it is connected or not.  This became a series in v2 to add flag
> setting to libxenguest.
> 
> Jason Andryuk (3):
>    xenconsole: Add connection flag
>    libs/guest: Set console page to disconnected
>    libs/guest: Set console as disconnected on resume
> 
>   tools/console/daemon/io.c                |  4 +++
>   tools/include/xenguest.h                 |  4 +++
>   tools/libs/guest/xg_dom_arm.c            |  2 +-
>   tools/libs/guest/xg_dom_boot.c           | 36 ++++++++++++++++++++++++
>   tools/libs/guest/xg_dom_x86.c            |  6 ++--
>   tools/libs/guest/xg_sr_restore_x86_hvm.c |  2 +-
>   tools/libs/guest/xg_sr_restore_x86_pv.c  |  1 +
>   xen/include/public/io/console.h          | 13 +++++++++
>   8 files changed, 63 insertions(+), 5 deletions(-)
> 

For the series:

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

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Re: [PATCH v2 0/3] xenconsole: Add connection flag

[Anthony PERARD]

On Tue, Sep 09, 2025 at 12:18:26PM +0200, Jürgen Groß wrote:
> On 22.08.25 23:39, Jason Andryuk wrote:
> > Add a connection flag to the console interface page so a domain can tell
> > if it is connected or not.  This became a series in v2 to add flag
> > setting to libxenguest.
> > 
> > Jason Andryuk (3):
> >    xenconsole: Add connection flag
> >    libs/guest: Set console page to disconnected
> >    libs/guest: Set console as disconnected on resume
> > 
> >   tools/console/daemon/io.c                |  4 +++
> >   tools/include/xenguest.h                 |  4 +++
> >   tools/libs/guest/xg_dom_arm.c            |  2 +-
> >   tools/libs/guest/xg_dom_boot.c           | 36 ++++++++++++++++++++++++
> >   tools/libs/guest/xg_dom_x86.c            |  6 ++--
> >   tools/libs/guest/xg_sr_restore_x86_hvm.c |  2 +-
> >   tools/libs/guest/xg_sr_restore_x86_pv.c  |  1 +
> >   xen/include/public/io/console.h          | 13 +++++++++
> >   8 files changed, 63 insertions(+), 5 deletions(-)
> > 
> 
> For the series:
> 
> Reviewed-by: Juergen Gross <jgross@suse.com>

For the series:
Acked-by: Anthony PERARD <anthony.perard@vates.tech>

Hi Oleksii,
I think this series needs your "release-ack" tag.

Thanks,

-- 
Anthony PERARD

Re: [PATCH v2 0/3] xenconsole: Add connection flag

[Oleksii Kurochko]

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On 9/29/25 3:21 PM, Anthony PERARD wrote:
> On Tue, Sep 09, 2025 at 12:18:26PM +0200, Jürgen Groß wrote:
>> On 22.08.25 23:39, Jason Andryuk wrote:
>>> Add a connection flag to the console interface page so a domain can tell
>>> if it is connected or not.  This became a series in v2 to add flag
>>> setting to libxenguest.
>>>
>>> Jason Andryuk (3):
>>>     xenconsole: Add connection flag
>>>     libs/guest: Set console page to disconnected
>>>     libs/guest: Set console as disconnected on resume
>>>
>>>    tools/console/daemon/io.c                |  4 +++
>>>    tools/include/xenguest.h                 |  4 +++
>>>    tools/libs/guest/xg_dom_arm.c            |  2 +-
>>>    tools/libs/guest/xg_dom_boot.c           | 36 ++++++++++++++++++++++++
>>>    tools/libs/guest/xg_dom_x86.c            |  6 ++--
>>>    tools/libs/guest/xg_sr_restore_x86_hvm.c |  2 +-
>>>    tools/libs/guest/xg_sr_restore_x86_pv.c  |  1 +
>>>    xen/include/public/io/console.h          | 13 +++++++++
>>>    8 files changed, 63 insertions(+), 5 deletions(-)
>>>
>> For the series:
>>
>> Reviewed-by: Juergen Gross<jgross@suse.com>
> For the series:
> Acked-by: Anthony PERARD<anthony.perard@vates.tech>
>
> Hi Oleksii,
> I think this series needs your "release-ack" tag.\

It is a little bit too late. But considering that this patch should increase
boot performance (right?) and it is pretty straightforward, I think (IIUC regarding
boot performance) we can consider these patch series to be in 4.21:
  Release-Acked-By: Oleksii Kurochko<oleksii.kurochko@gmail.com>

~ Oleksii

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