vmm: Add a "discard_writes=" to --pmem

This opens the backing file read-only, makes the pages in the mmap()
read-only and also makes the KVM mapping read-only. The file is also
mapped with MAP_PRIVATE to make the changes local to this process only.

This is functional alternative to having support for making a
virtio-pmem device readonly. Unfortunately there is no concept of
readonly virtio-pmem (or any type of NVDIMM/PMEM) in the Linux kernel so
to be able to have a block device that is appears readonly in the guest
requires significant specification and kernel changes.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

src/main.rs

@@ -173,7 +173,7 @@ fn create_app<'a, 'b>(
                 .help(
                     "Persistent memory parameters \
                      \"file=<backing_file_path>,size=<persistent_memory_size>,iommu=on|off,\
-                     mergeable=on|off\"",
+                     mergeable=on|off,discard_writes=on|off,\"",
                 )
                 .takes_value(true)
                 .min_values(1)

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -410,6 +410,9 @@ components:
         mergeable:
           type: boolean
           default: false
+        discard_writes:
+          type: boolean
+          default: false
 
     ConsoleConfig:
       required:

vmm/src/config.rs

@@ -766,6 +766,8 @@ pub struct PmemConfig {
     pub iommu: bool,
     #[serde(default)]
     pub mergeable: bool,
+    #[serde(default)]
+    pub discard_writes: bool,
 }
 
 impl PmemConfig {
@@ -777,6 +779,7 @@ impl PmemConfig {
         let mut size_str: &str = "";
         let mut iommu_str: &str = "";
         let mut mergeable_str: &str = "";
+        let mut discard_writes_str: &str = "";
 
         for param in params_list.iter() {
             if param.starts_with("file=") {
@@ -787,6 +790,8 @@ impl PmemConfig {
                 iommu_str = &param[6..];
             } else if param.starts_with("mergeable=") {
                 mergeable_str = &param[10..];
+            } else if param.starts_with("discard_writes=") {
+                discard_writes_str = &param[15..];
             }
         }
 
@@ -799,6 +804,7 @@ impl PmemConfig {
             size: parse_size(size_str)?,
             iommu: parse_on_off(iommu_str)?,
             mergeable: parse_on_off(mergeable_str)?,
+            discard_writes: parse_on_off(discard_writes_str)?,
         })
     }
 }

vmm/src/device_manager.rs

@@ -27,7 +27,7 @@ use arch::layout::{APIC_START, IOAPIC_SIZE, IOAPIC_START};
 use devices::{ioapic, BusDevice, HotPlugNotificationFlags};
 use kvm_ioctls::*;
 use libc::TIOCGWINSZ;
-use libc::{MAP_NORESERVE, MAP_SHARED, O_RDONLY, O_TMPFILE, PROT_READ, PROT_WRITE};
+use libc::{MAP_NORESERVE, MAP_PRIVATE, MAP_SHARED, O_TMPFILE, PROT_READ, PROT_WRITE};
 #[cfg(feature = "pci_support")]
 use pci::{
     DeviceRelocation, PciBarRegionType, PciBus, PciConfigIo, PciConfigMmio, PciDevice, PciRoot,
@@ -1392,7 +1392,7 @@ impl DeviceManager {
 
                 let file = OpenOptions::new()
                     .read(true)
-                    .write(true)
+                    .write(!pmem_cfg.discard_writes)
                     .custom_flags(custom_flags)
                     .open(&pmem_cfg.file)
                     .map_err(DeviceManagerError::PmemFileOpen)?;
@@ -1406,12 +1406,17 @@ impl DeviceManager {
                 let mmap_region = MmapRegion::build(
                     Some(FileOffset::new(cloned_file, 0)),
                     size as usize,
-                    if pmem_cfg.readonly {
+                    if pmem_cfg.discard_writes {
                         PROT_READ
                     } else {
                         PROT_READ | PROT_WRITE
                     },
-                    MAP_NORESERVE | MAP_SHARED,
+                    MAP_NORESERVE
+                        | if pmem_cfg.discard_writes {
+                            MAP_PRIVATE
+                        } else {
+                            MAP_SHARED
+                        },
                 )
                 .map_err(DeviceManagerError::NewMmapRegion)?;
                 let addr: u64 = mmap_region.as_ptr() as u64;
@@ -1426,7 +1431,7 @@ impl DeviceManager {
                         size,
                         addr,
                         pmem_cfg.mergeable,
-                        false,
+                        pmem_cfg.discard_writes,
                     )
                     .map_err(DeviceManagerError::MemoryManager)?;