Introduce minidumps. Full physical memory crash dumps are still available

via the debug.minidump sysctl and tunable.

Traditional dumps store all physical memory.  This was once a good thing
when machines had a maximum of 64M of ram and 1GB of kvm.  These days,
machines often have many gigabytes of ram and a smaller amount of kvm.
libkvm+kgdb don't have a way to access physical ram that is not mapped
into kvm at the time of the crash dump, so the extra ram being dumped
is mostly wasted.

Minidumps invert the process.  Instead of dumping physical memory in
in order to guarantee that all of kvm's backing is dumped, minidumps
instead dump only memory that is actively mapped into kvm.

amd64 has a direct map region that things like UMA use.  Obviously we
cannot dump all of the direct map region because that is effectively
an old style all-physical-memory dump.  Instead, introduce a bitmap
and two helper routines (dump_add_page(pa) and dump_drop_page(pa)) that
allow certain critical direct map pages to be included in the dump.
uma_machdep.c's allocator is the intended consumer.

Dumps are a custom format.  At the very beginning of the file is a header,
then a copy of the message buffer, then the bitmap of pages present in
the dump, then the final level of the kvm page table trees (2MB mappings
are expanded into a 4K page mappings), then the sparse physical pages
according to the bitmap.  libkvm can now conveniently access the kvm
page table entries.

Booting my test 8GB machine, forcing it into ddb and forcing a dump
leads to a 48MB minidump.  While this is a best case, I expect minidumps
to be in the 100MB-500MB range.  Obviously, never larger than physical
memory of course.

minidumps are on by default.  It would want be necessary to turn them off
if it was necessary to debug corrupt kernel page table management as that
would mess up minidumps as well.

Both minidumps and regular dumps are supported on the same machine.

1 files changed, 6 insertions, 0 deletions

diff --git a/sys/amd64/include/md_var.h b/sys/amd64/include/md_var.h
index 0c0407c89687..4e421eea1654 100644
--- a/sys/amd64/include/md_var.h
+++ b/sys/amd64/include/md_var.h
@@ -53,6 +53,8 @@ extern	char	cpu_vendor[];
 extern	char	kstack[];
 extern	char	sigcode[];
 extern	int	szsigcode;
+extern	uint64_t *vm_page_dump;
+extern	int	vm_page_dump_size;
 
 extern	struct pcpu __pcpu[];
 
@@ -61,11 +63,14 @@ struct	thread;
 struct	reg;
 struct	fpreg;
 struct  dbreg;
+struct	dumperinfo;
 
 void	busdma_swi(void);
 void	cpu_setregs(void);
 void	doreti_iret(void) __asm(__STRING(doreti_iret));
 void	doreti_iret_fault(void) __asm(__STRING(doreti_iret_fault));
+void	dump_add_page(vm_paddr_t);
+void	dump_drop_page(vm_paddr_t);
 void	initializecpu(void);
 void	fillw(int /*u_short*/ pat, void *base, size_t cnt);
 void	fpstate_drop(struct thread *td);
@@ -75,5 +80,6 @@ void	pagecopy(void *from, void *to);
 void	pagezero(void *addr);
 void	setidt(int idx, alias_for_inthand_t *func, int typ, int dpl, int ist);
 int	user_dbreg_trap(void);
+void	minidumpsys(struct dumperinfo *);
 
 #endif /* !_MACHINE_MD_VAR_H_ */