linux-kernel - [PATCH] drivers/xen/hypervisor: Expose VM SIF flags to userspace

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Message-ID: <20221129150058.266943-1-per.bilse@citrix.com>
Date:   Tue, 29 Nov 2022 15:00:57 +0000
From:   Per Bilse <per.bilse@...rix.com>
To:     <linux-kernel@...r.kernel.org>
CC:     Per Bilse <per.bilse@...rix.com>, Juergen Gross <jgross@...e.com>,
        "Stefano Stabellini" <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jan Beulich <jbeulich@...e.com>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@...ts.xenproject.org>
Subject: [PATCH] drivers/xen/hypervisor: Expose VM SIF flags to userspace

/proc/xen is a legacy pseudo filesystem which predates Xen support
getting merged into Linux.  It has largely been replaced with more
normal locations for data (/sys/hypervisor/ for info, /dev/xen/ for
user devices).  We want to compile xenfs support out of the dom0 kernel.

There is one item which only exists in /proc/xen, namely
/proc/xen/capabilities with "control_d" being the signal of "you're in
the control domain".  This ultimately comes from the SIF flags provided
at VM start.

This patch exposes all SIF flags in /sys/hypervisor/properties/flags,
which will coexist with /proc/xen while dependencies are being migrated.
Possible values are "privileged", "initdomain", "multiboot",
"mod_start_pfn", and "virtmap", with "initdomain" being the equivalent
of "control_d".

Signed-off-by: Per Bilse <per.bilse@...rix.com>
---
 drivers/xen/sys-hypervisor.c | 26 ++++++++++++++++++++++++++
 include/xen/interface/xen.h  | 13 ++++++++-----
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/drivers/xen/sys-hypervisor.c b/drivers/xen/sys-hypervisor.c
index fcb0792f090e..7393e04bdb6d 100644
--- a/drivers/xen/sys-hypervisor.c
+++ b/drivers/xen/sys-hypervisor.c
@@ -379,6 +379,31 @@ static ssize_t buildid_show(struct hyp_sysfs_attr *attr, char *buffer)
 
 HYPERVISOR_ATTR_RO(buildid);
 
+static ssize_t flags_show(struct hyp_sysfs_attr *attr, char *buffer)
+{
+	static char const *const sifstr[SIFN_NUM_SIFN] = {
+		[SIFN_PRIV]  = "privileged",
+		[SIFN_INIT]  = "initdomain",
+		[SIFN_MULTI] = "multiboot",
+		[SIFN_PFN]   = "mod_start_pfn",
+		[SIFN_VIRT]  = "virtmap"
+	};
+	unsigned sifnum, sifmask;
+	ssize_t ret = 0;
+
+	sifmask = ~(~0U << SIFN_NUM_SIFN);  // ...0000111...
+	if (xen_domain() && (xen_start_flags & sifmask) != 0) {
+		for (sifnum = 0; sifnum != SIFN_NUM_SIFN; sifnum++) {
+			if ((xen_start_flags & (1<<sifnum)) != 0)
+				ret += sprintf(buffer+ret, "%s ", sifstr[sifnum]);
+		}
+		buffer[ret-1] = '\n';
+	}
+	return ret;
+}
+
+HYPERVISOR_ATTR_RO(flags);
+
 static struct attribute *xen_properties_attrs[] = {
 	&capabilities_attr.attr,
 	&changeset_attr.attr,
@@ -386,6 +411,7 @@ static struct attribute *xen_properties_attrs[] = {
 	&pagesize_attr.attr,
 	&features_attr.attr,
 	&buildid_attr.attr,
+	&flags_attr.attr,
 	NULL
 };
 
diff --git a/include/xen/interface/xen.h b/include/xen/interface/xen.h
index 0ca23eca2a9c..762a348abe3e 100644
--- a/include/xen/interface/xen.h
+++ b/include/xen/interface/xen.h
@@ -648,11 +648,14 @@ struct start_info {
 };
 
 /* These flags are passed in the 'flags' field of start_info_t. */
-#define SIF_PRIVILEGED      (1<<0)  /* Is the domain privileged? */
-#define SIF_INITDOMAIN      (1<<1)  /* Is this the initial control domain? */
-#define SIF_MULTIBOOT_MOD   (1<<2)  /* Is mod_start a multiboot module? */
-#define SIF_MOD_START_PFN   (1<<3)  /* Is mod_start a PFN? */
-#define SIF_VIRT_P2M_4TOOLS (1<<4)  /* Do Xen tools understand a virt. mapped */
+/* Text strings are printed out in sys-hypervisor.c, we guard   */
+/* against mix-ups and errors by enumerating the flags.         */
+enum { SIFN_PRIV, SIFN_INIT, SIFN_MULTI, SIFN_PFN, SIFN_VIRT, SIFN_NUM_SIFN };
+#define SIF_PRIVILEGED      (1<<SIFN_PRIV)  /* Is the domain privileged? */
+#define SIF_INITDOMAIN      (1<<SIFN_INIT)  /* Is this the initial control domain? */
+#define SIF_MULTIBOOT_MOD   (1<<SIFN_MULTI) /* Is mod_start a multiboot module? */
+#define SIF_MOD_START_PFN   (1<<SIFN_PFN)   /* Is mod_start a PFN? */
+#define SIF_VIRT_P2M_4TOOLS (1<<SIFN_VIRT)  /* Do Xen tools understand a virt. mapped */
 				    /* P->M making the 3 level tree obsolete? */
 #define SIF_PM_MASK       (0xFF<<8) /* reserve 1 byte for xen-pm options */
 
-- 
2.31.1