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Re: [Xen-devel] [RFC PATCH v2 1/3] x86/mm/tlb: Change __flush_tlb_one_user interface

On 26.08.19 18:38, Nadav Amit wrote:

On Aug 26, 2019, at 12:51 AM, Juergen Gross <jgross@xxxxxxxx> wrote:

On 24.08.19 00:52, Nadav Amit wrote:

__flush_tlb_one_user() currently flushes a single entry, and flushes it
both in the kernel and user page-tables, when PTI is enabled.
Change __flush_tlb_one_user() and related interfaces into
__flush_tlb_range() that flushes a range and does not flush the user
page-table.
This refactoring is needed for the next patch, but regardless makes
sense and has several advantages. First, only Xen-PV, which does not
use PTI, implements the paravirtual interface of flush_tlb_one_user() so
nothing is broken by separating the user and kernel page-table flushes,
and the interface is more intuitive.
Second, INVLPG can flush unrelated mappings, and it is also a
serializing instruction. It is better to have a tight loop that flushes
the entries.
Third, currently __flush_tlb_one_kernel() also flushes the user
page-tables, which is not needed. This allows to avoid this redundant
flush.
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
  arch/x86/include/asm/paravirt.h       |  5 ++--
  arch/x86/include/asm/paravirt_types.h |  3 ++-
  arch/x86/include/asm/tlbflush.h       | 24 +++++------------
  arch/x86/kernel/paravirt.c            |  7 ++---
  arch/x86/mm/tlb.c                     | 39 ++++++++++++++++++++++-----
  arch/x86/xen/mmu_pv.c                 | 21 +++++++++------
  6 files changed, 62 insertions(+), 37 deletions(-)

...

diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 48f7c7eb4dbc..ed68657f5e77 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -1325,22 +1325,27 @@ static noinline void xen_flush_tlb(void)
        preempt_enable();
  }
  -static void xen_flush_tlb_one_user(unsigned long addr)
+static void xen_flush_tlb_range(unsigned long start, unsigned long end,
+                               u8 stride_shift)
  {
        struct mmuext_op *op;
        struct multicall_space mcs;
-
-       trace_xen_mmu_flush_tlb_one_user(addr);
+       unsigned long addr;
        preempt_disable();
        mcs = xen_mc_entry(sizeof(*op));
        op = mcs.args;
-       op->cmd = MMUEXT_INVLPG_LOCAL;
-       op->arg1.linear_addr = addr & PAGE_MASK;
-       MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
  -     xen_mc_issue(PARAVIRT_LAZY_MMU);
+       for (addr = start; addr < end; addr += 1ul << stride_shift) {
+               trace_xen_mmu_flush_tlb_one_user(addr);
+
+               op->cmd = MMUEXT_INVLPG_LOCAL;
+               op->arg1.linear_addr = addr & PAGE_MASK;
+               MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
+
+               xen_mc_issue(PARAVIRT_LAZY_MMU);
+       }


For this kind of usage (a loop) you should:

- replace the call of xen_mc_entry() with xen_mc_batch()
- use xen_extend_mmuext_op() for each loop iteration
- call xen_mc_issue() after the loop

Additionally I'd like you to replace trace_xen_mmu_flush_tlb_one_user()
with trace_xen_mmu_flush_tlb_range() taking all three parameters and
keep it where it was (out of the loop).

The paravirt parts seem to be okay.


Thanks for the quick response. I don’t think the preempt_disable/enable() is
needed in this case, but I kept them. Does the following match what you had
in mind?

Yes, it does. BTW, preempt_disable/enable() is needed as xen_mc_batch() ... xen_mc_issue() is using a percpu buffer. Juergen


---
  arch/x86/xen/mmu_pv.c      | 25 ++++++++++++++-----------
  include/trace/events/xen.h | 18 ++++++++++++------
  2 files changed, 26 insertions(+), 17 deletions(-)

diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 48f7c7eb4dbc..faa01591df36 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -1325,20 +1325,23 @@ static noinline void xen_flush_tlb(void)
        preempt_enable();
  }

-static void xen_flush_tlb_one_user(unsigned long addr)

+static void xen_flush_tlb_range(unsigned long start, unsigned long end,
+                               u8 stride_shift)
  {
-       struct mmuext_op *op;
-       struct multicall_space mcs;
-
-       trace_xen_mmu_flush_tlb_one_user(addr);
+       struct mmuext_op op;
+       unsigned long addr;

preempt_disable();- mcs = xen_mc_entry(sizeof(*op));

-       op = mcs.args;
-       op->cmd = MMUEXT_INVLPG_LOCAL;
-       op->arg1.linear_addr = addr & PAGE_MASK;
-       MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
+       xen_mc_batch();
+       op.cmd = MMUEXT_INVLPG_LOCAL;
+
+       trace_xen_mmu_flush_tlb_range(start, end, stride_shift);
+
+       for (addr = start; addr < end; addr += 1ul << stride_shift) {
+               op.arg1.linear_addr = addr & PAGE_MASK;
+               xen_extend_mmuext_op(&op);
+       }

xen_mc_issue(PARAVIRT_LAZY_MMU);@@ -2394,7 +2397,7 @@ static const struct pv_mmu_ops xen_mmu_ops __initconst = {.flush_tlb_user = xen_flush_tlb,

        .flush_tlb_kernel = xen_flush_tlb,
-       .flush_tlb_one_user = xen_flush_tlb_one_user,
+       .flush_tlb_range = xen_flush_tlb_range,
        .flush_tlb_multi = xen_flush_tlb_multi,
        .tlb_remove_table = tlb_remove_table,

diff --git a/include/trace/events/xen.h b/include/trace/events/xen.h

index 546022acf160..32bf0a94b4d8 100644
--- a/include/trace/events/xen.h
+++ b/include/trace/events/xen.h
@@ -352,14 +352,20 @@ DECLARE_EVENT_CLASS(xen_mmu_pgd,
  DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_pin);
  DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_unpin);

-TRACE_EVENT(xen_mmu_flush_tlb_one_user,

-           TP_PROTO(unsigned long addr),
-           TP_ARGS(addr),
+TRACE_EVENT(xen_mmu_flush_tlb_range,
+           TP_PROTO(unsigned long start, unsigned long end,
+                    unsigned char stride_shift),
+           TP_ARGS(start, end, stride_shift),
            TP_STRUCT__entry(
-                   __field(unsigned long, addr)
+                   __field(unsigned long, start)
+                   __field(unsigned long, end)
+                   __field(unsigned char, stride_shift)
                    ),
-           TP_fast_assign(__entry->addr = addr),
-           TP_printk("addr %lx", __entry->addr)
+           TP_fast_assign(__entry->start = start;
+                          __entry->end = end;
+                          __entry->stride_shift = stride_shift),
+           TP_printk("start %lx end %lx stride_shift %d", __entry->start,
+                     __entry->end, __entry->stride_shift)
        );

TRACE_EVENT(xen_mmu_flush_tlb_multi,

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