[RFCv2 07/10] x86/mm: Handle tagged memory accesses from kernel threads

Kirill A. Shutemov
2022-05-11T02:30:24Z

When a kernel thread performs memory access on behalf of a process (like
in async I/O, io_uring, etc.) it has to respect tagging setup of the
process as user addresses can include tags.

Normally, LAM setup is per-thread and recorded in thread features, but
for this use case kernel also tracks LAM setup per-mm. mm->context.lam
would record LAM that allows the most tag bits among the threads of
the mm.

The info used by switch_mm_irqs_off() to construct CR3 if the task is
kernel thread. Thread featrues of the kernel thread get updated
according to mm->context.lam. It allows untagged_addr() to work
correctly.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/include/asm/mmu.h | 1 +
arch/x86/mm/tlb.c | 28 ++++++++++++++++++++++++++++
2 files changed, 29 insertions(+)

diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
index 5d7494631ea9..52f3749f14e8 100644
--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -40,6 +40,7 @@ typedef struct {

#ifdef CONFIG_X86_64
unsigned short flags;
+ u8 lam;
#endif

struct mutex lock;
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index f9fe71d1f42c..b320556e1c22 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -185,6 +185,34 @@ static u8 gen_lam(struct task_struct *tsk, struct mm_struct *mm)
if (!tsk)
return LAM_NONE;

+ if (tsk->flags & PF_KTHREAD) {
+ /*
+ * For kernel thread use the most permissive LAM
+ * used by the mm. It's required to handle kernel thread
+ * memory accesses on behalf of a process.
+ *
+ * Adjust thread flags accodringly, so untagged_addr() would
+ * work correctly.
+ */
+
+ tsk->thread.features &= ~(X86_THREAD_LAM_U48 |
+ X86_THREAD_LAM_U57);
+
+ switch (mm->context.lam) {
+ case LAM_NONE:
+ return LAM_NONE;
+ case LAM_U57:
+ tsk->thread.features |= X86_THREAD_LAM_U57;
+ return LAM_U57;
+ case LAM_U48:
+ tsk->thread.features |= X86_THREAD_LAM_U48;
+ return LAM_U48;
+ default:
+ WARN_ON_ONCE(1);
+ return LAM_NONE;
+ }
+ }
+
if (tsk->thread.features & X86_THREAD_LAM_U57)
return LAM_U57;
if (tsk->thread.features & X86_THREAD_LAM_U48)
--
2.35.1