Hi,

On 16/06/2020 10:35, Keqian Zhu wrote:
kvm_set_pte is called to replace a target PTE with a desired one.
We always do this without changing the desired one, but if dirty
status set by hardware is coverred, let caller know it.

Signed-off-by: Keqian Zhu <zhukeqi...@huawei.com>
---
  arch/arm64/kvm/mmu.c | 36 +++++++++++++++++++++++++++++++++++-
  1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 5ad87bce23c0..27407153121b 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -194,11 +194,45 @@ static void clear_stage2_pmd_entry(struct kvm *kvm, pmd_t 
*pmd, phys_addr_t addr
        put_page(virt_to_page(pmd));
  }
-static inline void kvm_set_pte(pte_t *ptep, pte_t new_pte)
+#ifdef CONFIG_ARM64_HW_AFDBM
+/**
+ * @ret: true if dirty status set by hardware is coverred.

NIT: s/coverred/covered/, this is in several places.

+ */
+static bool kvm_set_pte(pte_t *ptep, pte_t new_pte)
+{
+       pteval_t old_pteval, new_pteval, pteval;
+       bool old_logging, new_no_write;
+
+       old_logging = kvm_hw_dbm_enabled() && !pte_none(*ptep) &&
+                     kvm_s2pte_dbm(ptep);
+       new_no_write = pte_none(new_pte) || kvm_s2pte_readonly(&new_pte);
+
+       if (!old_logging || !new_no_write) {
+               WRITE_ONCE(*ptep, new_pte);
+               dsb(ishst);
+               return false;
+       }
+
+       new_pteval = pte_val(new_pte);
+       pteval = READ_ONCE(pte_val(*ptep));

This usage of *ptep looks wrong - it's read twice using READ_ONCE (once in kvm_s2pte_dbm()) and once without any decoration (in the pte_none() call). Which looks a bit dodgy and at the very least needs some justification. AFAICT you would be better taking a local copy and using that rather than reading from memory repeatedly.

+       do {
+               old_pteval = pteval;
+               pteval = cmpxchg_relaxed(&pte_val(*ptep), old_pteval, 
new_pteval);
+       } while (pteval != old_pteval);
This look appears to be reinventing xchg_relaxed(). Any reason we can't just use xchg_relaxed()? Also we had a dsb() after the WRITE_ONCE but you are using the _relaxed variant here. What is the justification for not having a barrier?

+
+       return !kvm_s2pte_readonly(&__pte(pteval));
+}
+#else
+/**
+ * @ret: true if dirty status set by hardware is coverred.
+ */
+static inline bool kvm_set_pte(pte_t *ptep, pte_t new_pte)
  {
        WRITE_ONCE(*ptep, new_pte);
        dsb(ishst);
+       return false;
  }
+#endif /* CONFIG_ARM64_HW_AFDBM */

You might be able to avoid this #ifdef by redefining old_logging as:

  old_logging = IS_ENABLED(CONFIG_ARM64_HW_AFDBM) && ...

I *think* the compiler should be able to kill the dead code and leave you with just the above when the config symbol is off.

Steve

static inline void kvm_set_pmd(pmd_t *pmdp, pmd_t new_pmd)
  {


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