Kernel: Implement Page Attribute Table (PAT) support and Write-Combine

This allows us to enable Write-Combine on e.g. framebuffers,
significantly improving performance on bare metal.

To keep things simple we right now only use one of up to three bits
(bit 7 in the PTE), which maps to the PA4 entry in the PAT MSR, which
we set to the Write-Combine mode on each CPU at boot time.

2 files changed, 18 insertions, 0 deletions

diff --git a/Kernel/Memory/Region.cpp b/Kernel/Memory/Region.cpp
index b7c741ec19..d3323fbc99 100644
--- a/Kernel/Memory/Region.cpp
+++ b/Kernel/Memory/Region.cpp
@@ -212,6 +212,8 @@ bool Region::map_individual_page_impl(size_t page_index)
             pte->set_writable(is_writable());
         if (Processor::current().has_feature(CPUFeature::NX))
             pte->set_execute_disabled(!is_executable());
+        if (Processor::current().has_feature(CPUFeature::PAT))
+            pte->set_pat(is_write_combine());
         pte->set_user_allowed(user_allowed);
     }
     return true;
@@ -311,6 +313,18 @@ void Region::remap()
         TODO();
 }
 
+ErrorOr<void> Region::set_write_combine(bool enable)
+{
+    if (enable && !Processor::current().has_feature(CPUFeature::PAT)) {
+        dbgln("PAT is not supported, implement MTRR fallback if available");
+        return Error::from_errno(ENOTSUP);
+    }
+
+    m_write_combine = enable;
+    remap();
+    return {};
+}
+
 void Region::clear_to_zero()
 {
     VERIFY(vmobject().is_anonymous());
diff --git a/Kernel/Memory/Region.h b/Kernel/Memory/Region.h
index 3c447296e7..7dc83a78cb 100644
--- a/Kernel/Memory/Region.h
+++ b/Kernel/Memory/Region.h
@@ -87,6 +87,9 @@ public:
     [[nodiscard]] bool is_mmap() const { return m_mmap; }
     void set_mmap(bool mmap) { m_mmap = mmap; }
 
+    [[nodiscard]] bool is_write_combine() const { return m_write_combine; }
+    ErrorOr<void> set_write_combine(bool);
+
     [[nodiscard]] bool is_user() const { return !is_kernel(); }
     [[nodiscard]] bool is_kernel() const { return vaddr().get() < USER_RANGE_BASE || vaddr().get() >= kernel_mapping_base; }
 
@@ -220,6 +223,7 @@ private:
     bool m_stack : 1 { false };
     bool m_mmap : 1 { false };
     bool m_syscall_region : 1 { false };
+    bool m_write_combine : 1 { false };
 
     IntrusiveRedBlackTreeNode<FlatPtr, Region, RawPtr<Region>> m_tree_node;
     IntrusiveListNode<Region> m_vmobject_list_node;