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vmm: device_manager: Make PMEM "discard_writes" mode true CoW

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The PMEM support has an option called "discard_writes" which when true
will prevent changes to the device from hitting the backing file. This
is trying to be the equivalent of "readonly" support of the block
device.

Previously the memory of the device was marked as KVM_READONLY. This
resulted in a trap when the guest attempted to write to it resulting a
VM exit (and recently a warning). This has a very detrimental effect on
the performance so instead make "discard_writes" truly CoW by mapping
the memory as `PROT_READ | PROT_WRITE` and using `MAP_PRIVATE` to
establish the CoW mapping.

Fixes: #1795

Signed-off-by: Rob Bradford <robert.bradford@intel.com>
This commit is contained in:
Rob Bradford 2020-10-02 09:51:38 +01:00 committed by Sebastien Boeuf
parent 6eeab85db0
commit 2d457ab974
1 changed files with 2 additions and 6 deletions
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8
vmm/src/device_manager.rs
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@ -2347,11 +2347,7 @@ impl DeviceManager {
let mmap_region = MmapRegion::build( let mmap_region = MmapRegion::build(
Some(FileOffset::new(cloned_file, 0)), Some(FileOffset::new(cloned_file, 0)),
region_size as usize, region_size as usize,
if pmem_cfg.discard_writes { PROT_READ | PROT_WRITE,
PROT_READ
} else {
PROT_READ | PROT_WRITE
},
MAP_NORESERVE MAP_NORESERVE
| if pmem_cfg.discard_writes { | if pmem_cfg.discard_writes {
MAP_PRIVATE MAP_PRIVATE
@ -2371,7 +2367,7 @@ impl DeviceManager {
region_size, region_size,
host_addr, host_addr,
pmem_cfg.mergeable, pmem_cfg.mergeable,
pmem_cfg.discard_writes, false,
) )
.map_err(DeviceManagerError::MemoryManager)?; .map_err(DeviceManagerError::MemoryManager)?;