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

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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.
This commit is contained in:
Tom 2022-01-25 17:35:34 -07:00 • committed by Idan Horowitz
parent 1abbe9b02c
commit 6e46e21c42
6 changed files with 40 additions and 0 deletions
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14
Kernel/Memory/Region.cpp
View file

@ -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());

4
Kernel/Memory/Region.h
View file

@ -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;