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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCH] x86/ctxt-switch: Document and improve GDT handling
write_full_gdt_ptes() has a latent bug. Using virt_to_mfn() and iterating
with (mfn + i) is wrong, because of PDX compression. The context switch path
only functions correctly because NR_RESERVED_GDT_PAGES is 1.
As this is exceedingly unlikely to change moving foward, drop the loop
rather than inserting a BUILD_BUG_ON(NR_RESERVED_GDT_PAGES != 1).
With the loop dropped, write_full_gdt_ptes() becomes more obviously a poor
name, so rename it to update_xen_slot_in_full_gdt().
Furthermore, calling virt_to_mfn() in the context switch path is a lot of
wasted cycles for a result which is constant after boot.
Begin by documenting how Xen handles the GDTs across context switch.
From this, we observe that load_full_gdt() is completely independent of the
current CPU, and load_default_gdt() only gets passed the current CPU regular
GDT.
Add two extra per-cpu variables which cache the L1e for the regular and compat
GDT, calculated in cpu_smpboot_alloc()/trap_init() as appropriate, so
update_xen_slot_in_full_gdt() doesn't need to waste time performing the same
calculation on every context switch.
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
Slightly RFC.
I'm fairly confident this is better, but Juergen says that the some of his
scheduling perf tests notice large difference from subtle changes in
__context_switch(), so it would be useful to get some numbers from this
change.
The delta from this change is:
add/remove: 2/0 grow/shrink: 1/1 up/down: 320/-127 (193)
Function old new delta
cpu_smpboot_callback 1152 1456 +304
per_cpu__gdt_table_l1e - 8 +8
per_cpu__compat_gdt_table_l1e - 8 +8
__context_switch 1238 1111 -127
Total: Before=3339227, After=3339420, chg +0.01%
I'm not overly happy about the special case in trap_init() but I can't think
of a better place to put this.
Also, it should now be very obvious to people that Xen's current GDT handling
for non-PV vcpus is a recipe subtle bugs, if we ever manage to execute a stray
mov/pop %sreg instruction. We really ought to have Xen's regular GDT in an
area where slots 0-13 are either mapped to the zero page, or not present, so
we don't risk loading a non-faulting garbage selector.
---
xen/arch/x86/domain.c | 52 ++++++++++++++++++++++++++++++----------------
xen/arch/x86/smpboot.c | 4 ++++
xen/arch/x86/traps.c | 10 +++++++++
xen/include/asm-x86/desc.h | 2 ++
4 files changed, 50 insertions(+), 18 deletions(-)
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 84cafbe558..147f96a09e 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1635,23 +1635,42 @@ static void _update_runstate_area(struct vcpu *v)
v->arch.pv.need_update_runstate_area = 1;
}
+/*
+ * Overview of Xen's GDTs.
+ *
+ * Xen maintains per-CPU compat and regular GDTs which are both a single page
+ * in size. Some content is specific to each CPU (the TSS, the per-CPU marker
+ * for #DF handling, and optionally the LDT). The compat and regular GDTs
+ * differ by the layout and content of the guest accessible selectors.
+ *
+ * The Xen selectors live from 0xe000 (slot 14 of 16), and need to always
+ * appear in this position for interrupt/exception handling to work.
+ *
+ * A PV guest may specify GDT frames of their own (slots 0 to 13). Room for a
+ * full GDT exists in the per-domain mappings.
+ *
+ * To schedule a PV vcpu, we point slot 14 of the guest's full GDT at the
+ * current CPU's compat or regular (as appropriate) GDT frame. This is so
+ * that the per-CPU parts still work correctly after switching pagetables and
+ * loading the guests full GDT into GDTR.
+ *
+ * To schedule Idle or HVM vcpus, we load a GDT base address which causes the
+ * regular per-CPU GDT frame to appear with selectors at the appropriate
+ * offset.
+ */
static inline bool need_full_gdt(const struct domain *d)
{
return is_pv_domain(d) && !is_idle_domain(d);
}
-static void write_full_gdt_ptes(seg_desc_t *gdt, const struct vcpu *v)
+static void update_xen_slot_in_full_gdt(const struct vcpu *v, unsigned int cpu)
{
- unsigned long mfn = virt_to_mfn(gdt);
- l1_pgentry_t *pl1e = pv_gdt_ptes(v);
- unsigned int i;
-
- for ( i = 0; i < NR_RESERVED_GDT_PAGES; i++ )
- l1e_write(pl1e + FIRST_RESERVED_GDT_PAGE + i,
- l1e_from_pfn(mfn + i, __PAGE_HYPERVISOR_RW));
+ l1e_write(pv_gdt_ptes(v) + FIRST_RESERVED_GDT_PAGE,
+ !is_pv_32bit_vcpu(v) ? per_cpu(gdt_table_l1e, cpu)
+ : per_cpu(compat_gdt_table_l1e, cpu));
}
-static void load_full_gdt(const struct vcpu *v, unsigned int cpu)
+static void load_full_gdt(const struct vcpu *v)
{
struct desc_ptr gdt_desc = {
.limit = LAST_RESERVED_GDT_BYTE,
@@ -1661,11 +1680,12 @@ static void load_full_gdt(const struct vcpu *v,
unsigned int cpu)
lgdt(&gdt_desc);
}
-static void load_default_gdt(const seg_desc_t *gdt, unsigned int cpu)
+static void load_default_gdt(unsigned int cpu)
{
struct desc_ptr gdt_desc = {
.limit = LAST_RESERVED_GDT_BYTE,
- .base = (unsigned long)(gdt - FIRST_RESERVED_GDT_ENTRY),
+ .base = (unsigned long)(per_cpu(gdt_table, cpu) -
+ FIRST_RESERVED_GDT_ENTRY),
};
lgdt(&gdt_desc);
@@ -1678,7 +1698,6 @@ static void __context_switch(void)
struct vcpu *p = per_cpu(curr_vcpu, cpu);
struct vcpu *n = current;
struct domain *pd = p->domain, *nd = n->domain;
- seg_desc_t *gdt;
ASSERT(p != n);
ASSERT(!vcpu_cpu_dirty(n));
@@ -1718,15 +1737,12 @@ static void __context_switch(void)
psr_ctxt_switch_to(nd);
- gdt = !is_pv_32bit_domain(nd) ? per_cpu(gdt_table, cpu) :
- per_cpu(compat_gdt_table, cpu);
-
if ( need_full_gdt(nd) )
- write_full_gdt_ptes(gdt, n);
+ update_xen_slot_in_full_gdt(n, cpu);
if ( need_full_gdt(pd) &&
((p->vcpu_id != n->vcpu_id) || !need_full_gdt(nd)) )
- load_default_gdt(gdt, cpu);
+ load_default_gdt(cpu);
write_ptbase(n);
@@ -1739,7 +1755,7 @@ static void __context_switch(void)
if ( need_full_gdt(nd) &&
((p->vcpu_id != n->vcpu_id) || !need_full_gdt(pd)) )
- load_full_gdt(n, cpu);
+ load_full_gdt(n);
if ( pd != nd )
cpumask_clear_cpu(cpu, pd->dirty_cpumask);
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 730fe141fa..004285d14c 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -985,6 +985,8 @@ static int cpu_smpboot_alloc(unsigned int cpu)
if ( gdt == NULL )
goto out;
per_cpu(gdt_table, cpu) = gdt;
+ per_cpu(gdt_table_l1e, cpu) =
+ l1e_from_pfn(virt_to_mfn(gdt), __PAGE_HYPERVISOR_RW);
memcpy(gdt, boot_cpu_gdt_table, NR_RESERVED_GDT_PAGES * PAGE_SIZE);
BUILD_BUG_ON(NR_CPUS > 0x10000);
gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
@@ -992,6 +994,8 @@ static int cpu_smpboot_alloc(unsigned int cpu)
per_cpu(compat_gdt_table, cpu) = gdt = alloc_xenheap_pages(order,
memflags);
if ( gdt == NULL )
goto out;
+ per_cpu(compat_gdt_table_l1e, cpu) =
+ l1e_from_pfn(virt_to_mfn(gdt), __PAGE_HYPERVISOR_RW);
memcpy(gdt, boot_cpu_compat_gdt_table, NR_RESERVED_GDT_PAGES * PAGE_SIZE);
gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 8097ef3bf5..25b4b47e5e 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -97,7 +97,9 @@ DEFINE_PER_CPU(uint64_t, efer);
static DEFINE_PER_CPU(unsigned long, last_extable_addr);
DEFINE_PER_CPU_READ_MOSTLY(seg_desc_t *, gdt_table);
+DEFINE_PER_CPU_READ_MOSTLY(l1_pgentry_t, gdt_table_l1e);
DEFINE_PER_CPU_READ_MOSTLY(seg_desc_t *, compat_gdt_table);
+DEFINE_PER_CPU_READ_MOSTLY(l1_pgentry_t, compat_gdt_table_l1e);
/* Master table, used by CPU0. */
idt_entry_t __section(".bss.page_aligned") __aligned(PAGE_SIZE)
@@ -2059,6 +2061,14 @@ void __init trap_init(void)
}
}
+ /* Cache {,compat_}gdt_table_l1e now that physically relocation is done. */
+ this_cpu(gdt_table_l1e) =
+ l1e_from_pfn(virt_to_mfn(boot_cpu_gdt_table),
+ __PAGE_HYPERVISOR_RW);
+ this_cpu(compat_gdt_table_l1e) =
+ l1e_from_pfn(virt_to_mfn(boot_cpu_compat_gdt_table),
+ __PAGE_HYPERVISOR_RW);
+
percpu_traps_init();
cpu_init();
diff --git a/xen/include/asm-x86/desc.h b/xen/include/asm-x86/desc.h
index 85e83bcefb..e565727dc0 100644
--- a/xen/include/asm-x86/desc.h
+++ b/xen/include/asm-x86/desc.h
@@ -206,8 +206,10 @@ struct __packed desc_ptr {
extern seg_desc_t boot_cpu_gdt_table[];
DECLARE_PER_CPU(seg_desc_t *, gdt_table);
+DECLARE_PER_CPU(l1_pgentry_t, gdt_table_l1e);
extern seg_desc_t boot_cpu_compat_gdt_table[];
DECLARE_PER_CPU(seg_desc_t *, compat_gdt_table);
+DECLARE_PER_CPU(l1_pgentry_t, compat_gdt_table_l1e);
extern void load_TR(void);
--
2.11.0
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