common/host_memory: Implement a fallback if fastmem fails.

This falls back to the old approach of using a virtual buffer.

Windows is untested, but this build should fix support for Windows < 10 v1803. However without fastmem support at all.

src/common/host_memory.cpp

@@ -449,23 +449,54 @@ private:
     int fd{-1}; // memfd file descriptor, -1 is the error value of memfd_create
 };
 
-#else // ^^^ Linux ^^^
+#else // ^^^ Linux ^^^ vvv Generic vvv
 
-#error Please implement the host memory for your platform
+class HostMemory::Impl {
+public:
+    explicit Impl(size_t /*backing_size */, size_t /* virtual_size */) {
+        // This is just a place holder.
+        // Please implement fastmem in a propper way on your platform.
+        throw std::bad_alloc{};
+    }
 
-#endif
+    void Map(size_t virtual_offset, size_t host_offset, size_t length) {}
+
+    void Unmap(size_t virtual_offset, size_t length) {}
+
+    void Protect(size_t virtual_offset, size_t length, bool read, bool write) {}
+
+    u8* backing_base{nullptr};
+    u8* virtual_base{nullptr};
+};
+
+#endif // ^^^ Generic ^^^
 
 HostMemory::HostMemory(size_t backing_size_, size_t virtual_size_)
-    : backing_size(backing_size_),
-      virtual_size(virtual_size_), impl{std::make_unique<HostMemory::Impl>(
-                                       AlignUp(backing_size, PageAlignment),
-                                       AlignUp(virtual_size, PageAlignment) + 3 * HugePageSize)},
-      backing_base{impl->backing_base}, virtual_base{impl->virtual_base} {
+    : backing_size(backing_size_), virtual_size(virtual_size_) {
+    try {
+        // Try to allocate a fastmem arena.
+        // The implementation will fail with std::bad_alloc on errors.
+        impl = std::make_unique<HostMemory::Impl>(AlignUp(backing_size, PageAlignment),
+                                                  AlignUp(virtual_size, PageAlignment) +
+                                                      3 * HugePageSize);
+        backing_base = impl->backing_base;
+        virtual_base = impl->virtual_base;
+
+        if (virtual_base) {
             virtual_base += 2 * HugePageSize - 1;
             virtual_base -= reinterpret_cast<size_t>(virtual_base) & (HugePageSize - 1);
             virtual_base_offset = virtual_base - impl->virtual_base;
         }
 
+    } catch (const std::bad_alloc&) {
+        LOG_CRITICAL(HW_Memory,
+                     "Fastmem unavailable, falling back to VirtualBuffer for memory allocation");
+        fallback_buffer = std::make_unique<Common::VirtualBuffer<u8>>(backing_size);
+        backing_base = fallback_buffer->data();
+        virtual_base = nullptr;
+    }
+}
+
 HostMemory::~HostMemory() = default;
 
 HostMemory::HostMemory(HostMemory&&) noexcept = default;
@@ -478,7 +509,7 @@ void HostMemory::Map(size_t virtual_offset, size_t host_offset, size_t length) {
     ASSERT(length % PageAlignment == 0);
     ASSERT(virtual_offset + length <= virtual_size);
     ASSERT(host_offset + length <= backing_size);
-    if (length == 0) {
+    if (length == 0 || !virtual_base || !impl) {
         return;
     }
     impl->Map(virtual_offset + virtual_base_offset, host_offset, length);
@@ -488,7 +519,7 @@ void HostMemory::Unmap(size_t virtual_offset, size_t length) {
     ASSERT(virtual_offset % PageAlignment == 0);
     ASSERT(length % PageAlignment == 0);
     ASSERT(virtual_offset + length <= virtual_size);
-    if (length == 0) {
+    if (length == 0 || !virtual_base || !impl) {
         return;
     }
     impl->Unmap(virtual_offset + virtual_base_offset, length);
@@ -498,7 +529,7 @@ void HostMemory::Protect(size_t virtual_offset, size_t length, bool read, bool w
     ASSERT(virtual_offset % PageAlignment == 0);
     ASSERT(length % PageAlignment == 0);
     ASSERT(virtual_offset + length <= virtual_size);
-    if (length == 0) {
+    if (length == 0 || !virtual_base || !impl) {
         return;
     }
     impl->Protect(virtual_offset + virtual_base_offset, length, read, write);

src/common/host_memory.h

@@ -6,6 +6,7 @@
 
 #include <memory>
 #include "common/common_types.h"
+#include "common/virtual_buffer.h"
 
 namespace Common {
 
@@ -61,6 +62,9 @@ private:
     u8* backing_base{};
     u8* virtual_base{};
     size_t virtual_base_offset{};
+
+    // Fallback if fastmem is not supported on this platform
+    std::unique_ptr<Common::VirtualBuffer<u8>> fallback_buffer;
 };
 
 } // namespace Common