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Commit 5c2ffce1 authored by Anton Ivanov's avatar Anton Ivanov Committed by Richard Weinberger
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um: Revert to using stack for pt_regs in signal handling 



Reverts commit b6024b21 and
adjusts default stack sizing to cope with larger size of
floating point save registers on the newer Intel CPUs.

b6024b21 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: default avatarAnton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: default avatarRichard Weinberger <richard@nod.at>
parent 37624b58

arch/um/Kconfig

+7 −3
Original line number Diff line number Diff line
@@ -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"

arch/um/os-Linux/signal.c

+8 −20
Original line number Diff line number Diff line
@@ -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)