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[Qemu-devel] [PATCH 2/2] memory: Don't use memcpy for ram_device regions


From: Alex Williamson
Subject: [Qemu-devel] [PATCH 2/2] memory: Don't use memcpy for ram_device regions
Date: Mon, 24 Oct 2016 17:00:00 -0600
User-agent: StGit/0.17.1-dirty

With a vfio assigned device we lay down a base MemoryRegion registered
as an IO region, giving us read & write accessors.  If the region
supports mmap, we lay down a higher priority sub-region MemoryRegion
on top of the base layer initialized as a RAM device pointer to the
mmap.  Finally, if we have any quirks for the device (ie. address
ranges that need additional virtualization support), we put another IO
sub-region on top of the mmap MemoryRegion.  When this is flattened,
we now potentially have sub-page mmap MemoryRegions exposed which
cannot be directly mapped through KVM.

This is as expected, but a subtle detail of this is that we end up
with two different access mechanisms through QEMU.  If we disable the
mmap MemoryRegion, we make use of the IO MemoryRegion and service
accesses using pread and pwrite to the vfio device file descriptor.
If the mmap MemoryRegion is enabled and results in one of these
sub-page gaps, QEMU handles the access as RAM, using memcpy to the
mmap.  Using either pread/pwrite or the mmap directly should be
correct, but using memcpy causes us problems.  I expect that not only
does memcpy not necessarily honor the original width and alignment in
performing a copy, but it potentially also uses processor instructions
not intended for MMIO spaces.  It turns out that this has been a
problem for Realtek NIC assignment, which has such a quirk that
creates a sub-page mmap MemoryRegion access.

To resolve this, we disable memory_access_is_direct() for ram_device
regions since QEMU assumes that it can use memcpy for those regions.
Instead we access through MemoryRegionOps, which replaces the memcpy
with simple de-references of standard sizes to the host memory.  This
also allows us to eliminate the ram_device bool from the MemoryRegion
structure since we can simply test the ops pointer.

With this patch we attempt to provide unrestricted access to the RAM
device, allowing byte through qword access as well as unaligned
access.  The assumption here is that accesses initiated by the VM are
driven by a device specific driver, which knows the device
capabilities.  If unaligned accesses are not supported by the device,
we don't want them to work in a VM by performing multiple aligned
accesses to compose the unaligned access.  A down-side of this
philosophy is that the xp command from the monitor attempts to use
the largest available access weidth, unaware of the underlying
device.  Using memcpy had this same restriction, but at least now an
operator can dump individual registers, even if blocks of device
memory may result in access widths beyond the capabilities of a
given device (RTL NICs only support up to dword).

Reported-by: Thorsten Kohfeldt <address@hidden>
Signed-off-by: Alex Williamson <address@hidden>
---
 include/exec/memory.h |    7 +++--
 memory.c              |   70 ++++++++++++++++++++++++++++++++++++++++++++++++-
 trace-events          |    2 +
 3 files changed, 74 insertions(+), 5 deletions(-)

diff --git a/include/exec/memory.h b/include/exec/memory.h
index a75b8c3..2d4a287 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -209,7 +209,6 @@ struct MemoryRegion {
     void (*destructor)(MemoryRegion *mr);
     uint64_t align;
     bool terminates;
-    bool ram_device;
     bool enabled;
     bool warning_printed; /* For reservations */
     uint8_t vga_logging_count;
@@ -1480,9 +1479,11 @@ void *qemu_map_ram_ptr(RAMBlock *ram_block, ram_addr_t 
addr);
 static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
 {
     if (is_write) {
-        return memory_region_is_ram(mr) && !mr->readonly;
+        return memory_region_is_ram(mr) &&
+               !mr->readonly && !memory_region_is_ram_device(mr);
     } else {
-        return memory_region_is_ram(mr) || memory_region_is_romd(mr);
+        return (memory_region_is_ram(mr) && !memory_region_is_ram_device(mr)) 
||
+               memory_region_is_romd(mr);
     }
 }
 
diff --git a/memory.c b/memory.c
index 7ffcff1..d07f785 100644
--- a/memory.c
+++ b/memory.c
@@ -1128,6 +1128,71 @@ const MemoryRegionOps unassigned_mem_ops = {
     .endianness = DEVICE_NATIVE_ENDIAN,
 };
 
+static uint64_t memory_region_ram_device_read(void *opaque,
+                                              hwaddr addr, unsigned size)
+{
+    MemoryRegion *mr = opaque;
+    uint64_t data = (uint64_t)~0;
+
+    switch (size) {
+    case 1:
+        data = *(uint8_t *)(mr->ram_block->host + addr);
+        break;
+    case 2:
+        data = *(uint16_t *)(mr->ram_block->host + addr);
+        break;
+    case 4:
+        data = *(uint32_t *)(mr->ram_block->host + addr);
+        break;
+    case 8:
+        data = *(uint64_t *)(mr->ram_block->host + addr);
+        break;
+    }
+
+    trace_memory_region_ram_device_read(get_cpu_index(), mr, addr, data, size);
+
+    return data;
+}
+
+static void memory_region_ram_device_write(void *opaque, hwaddr addr,
+                                           uint64_t data, unsigned size)
+{
+    MemoryRegion *mr = opaque;
+
+    trace_memory_region_ram_device_write(get_cpu_index(), mr, addr, data, 
size);
+
+    switch (size) {
+    case 1:
+        *(uint8_t *)(mr->ram_block->host + addr) = (uint8_t)data;
+        break;
+    case 2:
+        *(uint16_t *)(mr->ram_block->host + addr) = (uint16_t)data;
+        break;
+    case 4:
+        *(uint32_t *)(mr->ram_block->host + addr) = (uint32_t)data;
+        break;
+    case 8:
+        *(uint64_t *)(mr->ram_block->host + addr) = data;
+        break;
+    }
+}
+
+static const MemoryRegionOps ram_device_mem_ops = {
+    .read = memory_region_ram_device_read,
+    .write = memory_region_ram_device_write,
+    .endianness = DEVICE_NATIVE_ENDIAN,
+    .valid = {
+        .min_access_size = 1,
+        .max_access_size = 8,
+        .unaligned = true,
+    },
+    .impl = {
+        .min_access_size = 1,
+        .max_access_size = 8,
+        .unaligned = true,
+    },
+};
+
 bool memory_region_access_valid(MemoryRegion *mr,
                                 hwaddr addr,
                                 unsigned size,
@@ -1362,7 +1427,8 @@ void memory_region_init_ram_device_ptr(MemoryRegion *mr,
                                        void *ptr)
 {
     memory_region_init_ram_ptr(mr, owner, name, size, ptr);
-    mr->ram_device = true;
+    mr->ops = &ram_device_mem_ops;
+    mr->opaque = mr;
 }
 
 void memory_region_init_alias(MemoryRegion *mr,
@@ -1498,7 +1564,7 @@ const char *memory_region_name(const MemoryRegion *mr)
 
 bool memory_region_is_ram_device(MemoryRegion *mr)
 {
-    return mr->ram_device;
+    return mr->ops == &ram_device_mem_ops;
 }
 
 uint8_t memory_region_get_dirty_log_mask(MemoryRegion *mr)
diff --git a/trace-events b/trace-events
index 8ecded5..f74e1d3 100644
--- a/trace-events
+++ b/trace-events
@@ -121,6 +121,8 @@ memory_region_subpage_read(int cpu_index, void *mr, 
uint64_t offset, uint64_t va
 memory_region_subpage_write(int cpu_index, void *mr, uint64_t offset, uint64_t 
value, unsigned size) "cpu %d mr %p offset %#"PRIx64" value %#"PRIx64" size %u"
 memory_region_tb_read(int cpu_index, uint64_t addr, uint64_t value, unsigned 
size) "cpu %d addr %#"PRIx64" value %#"PRIx64" size %u"
 memory_region_tb_write(int cpu_index, uint64_t addr, uint64_t value, unsigned 
size) "cpu %d addr %#"PRIx64" value %#"PRIx64" size %u"
+memory_region_ram_device_read(int cpu_index, void *mr, uint64_t addr, uint64_t 
value, unsigned size) "cpu %d mr %p addr %#"PRIx64" value %#"PRIx64" size %u"
+memory_region_ram_device_write(int cpu_index, void *mr, uint64_t addr, 
uint64_t value, unsigned size) "cpu %d mr %p addr %#"PRIx64" value %#"PRIx64" 
size %u"
 
 ### Guest events, keep at bottom
 




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