On 09.09.25 11:37, Jürgen Groß wrote:
On 09.09.25 11:13, David Hildenbrand wrote:
On 08.09.25 09:39, Kevin Brodsky wrote:
Commit 49147beb0ccb ("x86/xen: allow nesting of same lazy mode")
originally introduced support for nested lazy sections (LAZY_MMU and
LAZY_CPU). It later got reverted by commit c36549ff8d84 as its
implementation turned out to be intolerant to preemption.
Now that the lazy_mmu API allows enter() to pass through a state to
the matching leave() call, we can support nesting again for the
LAZY_MMU mode in a preemption-safe manner. If xen_enter_lazy_mmu() is
called inside an active lazy_mmu section, xen_lazy_mode will already
be set to XEN_LAZY_MMU and we can then return LAZY_MMU_NESTED to
instruct the matching xen_leave_lazy_mmu() call to leave
xen_lazy_mode unchanged.
The only effect of this patch is to ensure that xen_lazy_mode
remains set to XEN_LAZY_MMU until the outermost lazy_mmu section
ends. xen_leave_lazy_mmu() still calls xen_mc_flush()
unconditionally.
Signed-off-by: Kevin Brodsky <kevin.brod...@arm.com>
---
arch/x86/include/asm/paravirt.h | 6 ++----
arch/x86/include/asm/paravirt_types.h | 4 ++--
arch/x86/xen/mmu_pv.c | 11 ++++++++---
3 files changed, 12 insertions(+), 9 deletions(-)
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 65a0d394fba1..4ecd3a6b1dea 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -529,14 +529,12 @@ static inline void arch_end_context_switch(struct
task_struct *next)
#define __HAVE_ARCH_ENTER_LAZY_MMU_MODE
static inline lazy_mmu_state_t arch_enter_lazy_mmu_mode(void)
{
- PVOP_VCALL0(mmu.lazy_mode.enter);
-
- return LAZY_MMU_DEFAULT;
+ return PVOP_CALL0(lazy_mmu_state_t, mmu.lazy_mode.enter);
}
static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)
{
- PVOP_VCALL0(mmu.lazy_mode.leave);
+ PVOP_VCALL1(mmu.lazy_mode.leave, state);
}
static inline void arch_flush_lazy_mmu_mode(void)
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/
paravirt_types.h
index bc1af86868a3..b7c567ccbf32 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -45,8 +45,8 @@ typedef int lazy_mmu_state_t;
struct pv_lazy_ops {
/* Set deferred update mode, used for batching operations. */
- void (*enter)(void);
- void (*leave)(void);
+ lazy_mmu_state_t (*enter)(void);
+ void (*leave)(lazy_mmu_state_t);
void (*flush)(void);
} __no_randomize_layout;
#endif
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 2039d5132ca3..6e5390ff06a5 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -2130,9 +2130,13 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t
phys, pgprot_t prot)
#endif
}
-static void xen_enter_lazy_mmu(void)
+static lazy_mmu_state_t xen_enter_lazy_mmu(void)
{
+ if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU)
+ return LAZY_MMU_NESTED;
+
You mention above "preemption-safe manner" above, so I am wondering,
what if we get preempted immediately after doing the this_cpu_read() and get
scheduled on another CPU?
This should still be correct: preemption needs a context switch to happen,
so xen_start_context_switch() and xen_end_context_switch() are involved.
Those are dealing with this problem by doing the right thing in the old
and the new context.
Thanks, that makes sense. Would be valuable to add that detail to the
patch description.
--
Cheers
David / dhildenb
