[Xen-devel] [PATCH] x86/vtx: Improvements to ept= command line handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Switch parse_ept_param() to use the parse_boolean() infrastructure for more
consistency with related command line parameters.  Rename opt_pml_enabled to
opt_ept_pml for consistency with opt_ept_ad, and switch it to being bool

Drop the comment leading comment for parse_ept_param().  It is stale, and just
repeats the command line documentation.

For the command line documentation, rewrite it largely from scratch, updating
to the latest metadata style.  Document A/D first, including a note about
AVR41, and modify PML to note its dependency on A/D.

No practical changes to behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>
---
 docs/misc/xen-command-line.markdown | 40 ++++++++++++++++++++++---------------
 xen/arch/x86/hvm/vmx/vmcs.c         | 28 ++++++++------------------
 2 files changed, 32 insertions(+), 36 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index 44ee51a..78b207c 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -841,29 +841,37 @@ effect the inverse meaning.
 >> Allows mapping of RuntimeServices which have no cachability attribute
 >> set as UC.
 
-### ept (Intel)
-> `= List of ( {no-}pml | {no-}ad )`
+### ept
+> `= List of [ ad=<bool>, pml=<bool> ]`
 
-Controls EPT related features.
+> Applicability: Intel
 
-> Sub-options:
-
-> `pml`
+Extended Page Tables are a feature of Intel's VT-x technology, whereby
+hardware manages the virtualisation of HVM guest pagetables.  EPT was
+introduced with the Nehalem architecture.
 
-> Default: `true`
+*   The `ad` boolean controls hardware tracking of Access and Dirty bits in the
+    EPT pagetables, and was first introduced in Broadwell Server.
 
->> PML is a new hardware feature in Intel's Broadwell Server and further
->> platforms which reduces hypervisor overhead of log-dirty mechanism by
->> automatically recording GPAs (guest physical addresses) when guest memory
->> gets dirty, and therefore significantly reducing number of EPT violation
->> caused by write protection of guest memory, which is a necessity to
->> implement log-dirty mechanism before PML.
+    By default, Xen will use A/D tracking when available in hardware, except
+    on Avoton processors affected by erratum AVR41.  Explicitly choosing
+    `ad=0` will disable the use of A/D tracking on capable hardware, whereas
+    choosing `ad=1` will cause tracking to be used even on AVR41-affected
+    hardware.
 
-> `ad`
+*   The `pml` boolean controls the use of Page Modification Logging, which is
+    also introduced in Broadwell Server.
 
-> Default: Hardware dependent
+    PML is a feature whereby the processor generates a list of pages which
+    have been dirtied.  This is necessary information for operations such as
+    live migration, and having the processor maintain the list of dirtied
+    pages is more efficient than traditional software implementations where
+    all guest writes trap into Xen so the dirty bitmap can be maintained.
 
->> Have hardware keep accessed/dirty (A/D) bits updated.
+    By default, Xen will use PML when it is available in hardware.  PML
+    functionally depends on A/D tracking, so choosing `ad=0` will implicitly
+    disable PML.  `pml=0` can be used to prevent the use of PML on otherwise
+    capable hardware.
 
 ### extra\_guest\_irqs
 > `= [<domU number>][,<dom0 number>]`
diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
index d6366c2..74f2a08 100644
--- a/xen/arch/x86/hvm/vmx/vmcs.c
+++ b/xen/arch/x86/hvm/vmx/vmcs.c
@@ -65,35 +65,23 @@ integer_param("ple_gap", ple_gap);
 static unsigned int __read_mostly ple_window = 4096;
 integer_param("ple_window", ple_window);
 
-static bool_t __read_mostly opt_pml_enabled = 1;
+static bool __read_mostly opt_ept_pml = true;
 static s8 __read_mostly opt_ept_ad = -1;
 
-/*
- * The 'ept' parameter controls functionalities that depend on, or impact the
- * EPT mechanism. Optional comma separated value may contain:
- *
- *  pml                 Enable PML
- *  ad                  Use A/D bits
- */
 static int __init parse_ept_param(const char *s)
 {
     const char *ss;
-    int rc = 0;
+    int val, rc = 0;
 
     do {
-        bool_t val = !!strncmp(s, "no-", 3);
-
-        if ( !val )
-            s += 3;
-
         ss = strchr(s, ',');
         if ( !ss )
             ss = strchr(s, '\0');
 
-        if ( !strncmp(s, "pml", ss - s) )
-            opt_pml_enabled = val;
-        else if ( !strncmp(s, "ad", ss - s) )
+        if ( (val = parse_boolean("ad", s, ss)) >= 0 )
             opt_ept_ad = val;
+        else if ( (val = parse_boolean("pml", s, ss)) >= 0 )
+            opt_ept_pml = val;
         else
             rc = -EINVAL;
 
@@ -247,7 +235,7 @@ static int vmx_init_vmcs_config(void)
             opt |= SECONDARY_EXEC_ENABLE_VPID;
         if ( opt_unrestricted_guest_enabled )
             opt |= SECONDARY_EXEC_UNRESTRICTED_GUEST;
-        if ( opt_pml_enabled )
+        if ( opt_ept_pml )
             opt |= SECONDARY_EXEC_ENABLE_PML;
 
         /*
@@ -330,9 +318,9 @@ static int vmx_init_vmcs_config(void)
     if ( !(_vmx_secondary_exec_control & SECONDARY_EXEC_ENABLE_EPT) )
         _vmx_secondary_exec_control &= ~SECONDARY_EXEC_ENABLE_PML;
 
-    /* Turn off opt_pml_enabled if PML feature is not present */
+    /* Turn off opt_ept_pml if PML feature is not present. */
     if ( !(_vmx_secondary_exec_control & SECONDARY_EXEC_ENABLE_PML) )
-        opt_pml_enabled = 0;
+        opt_ept_pml = false;
 
     if ( (_vmx_secondary_exec_control & SECONDARY_EXEC_PAUSE_LOOP_EXITING) &&
           ple_gap == 0 )
-- 
2.1.4


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