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um: Revert to using stack for pt_regs in signal handling

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Reverts commit b6024b21fe and
adjusts default stack sizing to cope with larger size of
floating point save registers on the newer Intel CPUs.

b6024b21fe replaced storing the
register state on the stack with kmalloc-ed storage. That has
a number of issues and a panic if that fails.
    1. kmalloc/ATOMIC can fail. There was a latent hard crash
in all interrupt and fault handling as a result.
    2. kmalloc in the interrupt path introduces a considerable
performance penalty for networking ~ 14% on iperf.

This commit restores uml to a stable state until a better
solution is found.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
This commit is contained in:
Anton Ivanov 2019-01-04 15:38:21 +00:00 committed by Richard Weinberger
parent 37624b5854
commit 5c2ffce1e9
2 changed files with 15 additions and 23 deletions
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10
arch/um/Kconfig
View file

@ -142,13 +142,17 @@ config MAGIC_SYSRQ
config KERNEL_STACK_ORDER
int "Kernel stack size order"
default 1 if 64BIT
range 1 10 if 64BIT
default 0 if !64BIT
default 2 if 64BIT
range 2 10 if 64BIT
default 1 if !64BIT
help
This option determines the size of UML kernel stacks. They will
be 1 << order pages. The default is OK unless you're running Valgrind
on UML, in which case, set this to 3.
It is possible to reduce the stack to 1 for 64BIT and 0 for 32BIT on
older (pre-2017) CPUs. It is not recommended on newer CPUs due to the
increase in the size of the state which needs to be saved when handling
signals.
config MMAPPER
tristate "iomem emulation driver"

28
arch/um/os-Linux/signal.c
View file

@ -31,29 +31,23 @@ void (*sig_info[NSIG])(int, struct siginfo *, struct uml_pt_regs *) = {
static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)
{
struct uml_pt_regs *r;
struct uml_pt_regs r;
int save_errno = errno;
r = uml_kmalloc(sizeof(struct uml_pt_regs), UM_GFP_ATOMIC);
if (!r)
panic("out of memory");
r->is_user = 0;
r.is_user = 0;
if (sig == SIGSEGV) {
/* For segfaults, we want the data from the sigcontext. */
get_regs_from_mc(r, mc);
GET_FAULTINFO_FROM_MC(r->faultinfo, mc);
get_regs_from_mc(&r, mc);
GET_FAULTINFO_FROM_MC(r.faultinfo, mc);
}
/* enable signals if sig isn't IRQ signal */
if ((sig != SIGIO) && (sig != SIGWINCH) && (sig != SIGALRM))
unblock_signals();
(*sig_info[sig])(sig, si, r);
(*sig_info[sig])(sig, si, &r);
errno = save_errno;
free(r);
}
/*
@ -91,17 +85,11 @@ void sig_handler(int sig, struct siginfo *si, mcontext_t *mc)
static void timer_real_alarm_handler(mcontext_t *mc)
{
struct uml_pt_regs *regs;
regs = uml_kmalloc(sizeof(struct uml_pt_regs), UM_GFP_ATOMIC);
if (!regs)
panic("out of memory");
struct uml_pt_regs regs;
if (mc != NULL)
get_regs_from_mc(regs, mc);
timer_handler(SIGALRM, NULL, regs);
free(regs);
get_regs_from_mc(&regs, mc);
timer_handler(SIGALRM, NULL, &regs);
}
void timer_alarm_handler(int sig, struct siginfo *unused_si, mcontext_t *mc)