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Asm versions of Xen pv-ops, suitable for either direct use or inlining.
The inline versions are the same as the direct-use versions, with the
pre- and post-amble chopped off.
This code is encoded for size rather than absolute efficiency,
with a view to being able to inline as much as possible.
We only bother with direct forms (ie, vcpu in pda) of the operations
here; the indirect forms are better handled in C, since they're
generally too large to inline anyway.
#include <linux/linkage.h>
#include <asm/asm-offsets.h>
#include <asm/thread_info.h>
#include <asm/percpu.h>
#include <asm/processor-flags.h>
#include <asm/segment.h>
#include <xen/interface/xen.h>
#define RELOC(x, v) .globl x##_reloc; x##_reloc=v
#define ENDPATCH(x) .globl x##_end; x##_end=.
/* Pseudo-flag used for virtual NMI, which we don't implement yet */
#define XEN_EFLAGS_NMI 0x80000000
Enable events. This clears the event mask and tests the pending
event status with one and operation. If there are pending
events, then enter the hypervisor to get them handled.
/* Unmask events */
movb $0, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
/* Preempt here doesn't matter because that will deal with
any pending interrupts. The pending check may end up being
run on the wrong CPU, but that doesn't hurt. */
/* Test for pending */
testb $0xff, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_pending
jz 1f
2: call check_events
RELOC(xen_irq_enable_direct, 2b+1)
Disabling events is simply a matter of making the event mask
movb $1, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
RELOC(xen_irq_disable_direct, 0)
(xen_)save_fl is used to get the current interrupt enable status.
Callers expect the status to be in X86_EFLAGS_IF, and other bits
may be set in the return value. We take advantage of this by
making sure that X86_EFLAGS_IF has the right value (and other bits
in that byte are 0), but other bits in the return value are
undefined. We need to toggle the state of the bit, because
Xen and x86 use opposite senses (mask vs enable).
testb $0xff, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
setz %ah
addb %ah,%ah
RELOC(xen_save_fl_direct, 0)
In principle the caller should be passing us a value return
from xen_save_fl_direct, but for robustness sake we test only
the X86_EFLAGS_IF flag rather than the whole byte. After
setting the interrupt mask state, it checks for unmasked
pending events and enters the hypervisor to get them delivered
if so.
testb $X86_EFLAGS_IF>>8, %ah
setz PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask
/* Preempt here doesn't matter because that will deal with
any pending interrupts. The pending check may end up being
run on the wrong CPU, but that doesn't hurt. */
/* check for unmasked and pending */
cmpw $0x0001, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_pending
jz 1f
2: call check_events
RELOC(xen_restore_fl_direct, 2b+1)
We can't use sysexit directly, because we're not running in ring0.
But we can easily fake it up using iret. Assuming xen_sysexit
is jumped to with a standard stack frame, we can just strip it
back to a standard iret frame and use iret.
movl PT_EAX(%esp), %eax /* Shouldn't be necessary? */
orl $X86_EFLAGS_IF, PT_EFLAGS(%esp)
lea PT_EIP(%esp), %esp
jmp xen_iret
This is run where a normal iret would be run, with the same stack setup:
8: eflags
4: cs
esp-> 0: eip
This attempts to make sure that any pending events are dealt
with on return to usermode, but there is a small window in
which an event can happen just before entering usermode. If
the nested interrupt ends up setting one of the TIF_WORK_MASK
pending work flags, they will not be tested again before
returning to usermode. This means that a process can end up
with pending work, which will be unprocessed until the process
enters and leaves the kernel again, which could be an
unbounded amount of time. This means that a pending signal or
reschedule event could be indefinitely delayed.
The fix is to notice a nested interrupt in the critical
window, and if one occurs, then fold the nested interrupt into
the current interrupt stack frame, and re-process it
iteratively rather than recursively. This means that it will
exit via the normal path, and all pending work will be dealt
with appropriately.
Because the nested interrupt handler needs to deal with the
current stack state in whatever form its in, we keep things
simple by only using a single register which is pushed/popped
on the stack.
/* test eflags for special cases */
testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
jnz hyper_iret
push %eax
ESP_OFFSET=4 # bytes pushed onto stack
/* Store vcpu_info pointer for easy access. Do it this
way to avoid having to reload %fs */
movl TI_cpu(%eax),%eax
movl __per_cpu_offset(,%eax,4),%eax
mov per_cpu__xen_vcpu(%eax),%eax
movl per_cpu__xen_vcpu, %eax
/* check IF state we're restoring */
testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp)
/* Maybe enable events. Once this happens we could get a
recursive event, so the critical region starts immediately
afterwards. However, if that happens we don't end up
resuming the code, so we don't have to be worried about
being preempted to another CPU. */
setz XEN_vcpu_info_mask(%eax)
/* check for unmasked and pending */
cmpw $0x0001, XEN_vcpu_info_pending(%eax)
/* If there's something pending, mask events again so we
can jump back into xen_hypervisor_callback */
sete XEN_vcpu_info_mask(%eax)
popl %eax
/* From this point on the registers are restored and the stack
updated, so we don't need to worry about it if we're preempted */
/* Jump to hypervisor_callback after fixing up the stack.
Events are masked, so jumping out of the critical
region is OK. */
je xen_hypervisor_callback
1: iret
.section __ex_table,"a"
.align 4
.long 1b,iret_exc
/* put this out of line since its very rarely used */
jmp hypercall_page + __HYPERVISOR_iret * 32
.globl xen_iret_start_crit, xen_iret_end_crit
This is called by xen_hypervisor_callback in entry.S when it sees
that the EIP at the time of interrupt was between xen_iret_start_crit
and xen_iret_end_crit. We're passed the EIP in %eax so we can do
a more refined determination of what to do.
The stack format at this point is:
ss : (ss/esp may be present if we came from usermode)
esp :
eflags } outer exception info
cs }
eip }
---------------- <- edi (copy dest)
eax : outer eax if it hasn't been restored
eflags } nested exception info
cs } (no ss/esp because we're nested
eip } from the same ring)
orig_eax }<- esi (copy src)
- - - - - - - -
fs }
es }
ds } SAVE_ALL state
eax }
: :
ebx }<- esp
In order to deliver the nested exception properly, we need to shift
everything from the return addr up to the error code so it
sits just under the outer exception info. This means that when we
handle the exception, we do it in the context of the outer exception
rather than starting a new one.
The only caveat is that if the outer eax hasn't been
restored yet (ie, it's still on stack), we need to insert
its value into the SAVE_ALL state before going on, since
it's usermode state which we eventually need to restore.
Paranoia: Make sure we're really coming from kernel space.
One could imagine a case where userspace jumps into the
critical range address, but just before the CPU delivers a GP,
it decides to deliver an interrupt instead. Unlikely?
Definitely. Easy to avoid? Yes. The Intel documents
explicitly say that the reported EIP for a bad jump is the
jump instruction itself, not the destination, but some virtual
environments get this wrong.
movl PT_CS(%esp), %ecx
andl $SEGMENT_RPL_MASK, %ecx
cmpl $USER_RPL, %ecx
je 2f
lea PT_ORIG_EAX(%esp), %esi
lea PT_EFLAGS(%esp), %edi
/* If eip is before iret_restore_end then stack
hasn't been restored yet. */
cmp $iret_restore_end, %eax
jae 1f
movl 0+4(%edi),%eax /* copy EAX (just above top of frame) */
movl %eax, PT_EAX(%esp)
lea ESP_OFFSET(%edi),%edi /* move dest up over saved regs */
/* set up the copy */
1: std
mov $PT_EIP / 4, %ecx /* saved regs up to orig_eax */
rep movsl
lea 4(%edi),%esp /* point esp to new frame */
2: jmp xen_do_upcall
Force an event check by making a hypercall,
but preserve regs before making the call.
push %eax
push %ecx
push %edx
call force_evtchn_callback
pop %edx
pop %ecx
pop %eax