On Tue, Aug 14, 2012 at 10:58:07AM +0800, Xiao Guangrong wrote:
> On 08/14/2012 01:39 AM, Marcelo Tosatti wrote:
> > On Sat, Aug 11, 2012 at 11:36:20AM +0800, Xiao Guangrong wrote:
> >> On 08/11/2012 02:14 AM, Marcelo Tosatti wrote:
> >>> On Tue, Aug 07, 2012 at 05:47:15PM +0800, Xiao Guangrong wrote:
> >>>> Changelog:
> >>>> - introduce KVM_PFN_ERR_RO_FAULT instead of dummy page
> >>>> - introduce KVM_HVA_ERR_BAD and optimize error hva indicators
> >>>>
> >>>> The test case can be found at:
> >>>> http://lkml.indiana.edu/hypermail/linux/kernel/1207.2/00819/migrate-perf.tar.bz2
> >>>>
> >>>> In current code, if we map a readonly memory space from host to guest
> >>>> and the page is not currently mapped in the host, we will get a fault-pfn
> >>>> and async is not allowed, then the vm will crash.
> >>>>
> >>>> As Avi's suggestion, We introduce readonly memory region to map ROM/ROMD
> >>>> to the guest, read access is happy for readonly memslot, write access on
> >>>> readonly memslot will cause KVM_EXIT_MMIO exit.
> >>>
> >>> Memory slots whose QEMU mapping is write protected is supported
> >>> today, as long as there are no write faults.
> >>>
> >>> What prevents the use of mmap(!MAP_WRITE) to handle read-only memslots
> >>> again?
> >>>
> >>
> >> It is happy to map !write host memory space to the readonly memslot,
> >> and they can coexist as well.
> >>
> >> readonly memslot checks the write-permission by seeing slot->flags and
> >> !write memory checks the write-permission in hva_to_pfn() function
> >> which checks vma->flags. It is no conflict.
> >
> > Yes, there is no conflict. The point is, if you can use the
> > mmap(PROT_READ) interface (supporting read faults on read-only slots)
> > for this behavior, what is the advantage of a new memslot flag?
> >
>
> You can get the discussion at:
> https://lkml.org/lkml/2012/5/22/228
>
> > I'm not saying mmap(PROT_READ) is the best interface, i am just asking
> > why it is not.
>
> My fault. :(
>
> >
> >>> The initial objective was to fix a vm crash, can you explain that
> >>> initial problem?
> >>>
> >>
> >> The issue was trigged by this code:
> >>
> >> } else {
> >> if (async && (vma->vm_flags & VM_WRITE))
> >> *async = true;
> >> pfn = KVM_PFN_ERR_FAULT;
> >> }
> >>
> >> If the host memory region is readonly (!vma->vm_flags & VM_WRITE) and
> >> its physical page is swapped out (or the file data does not be read in),
> >> get_user_page_nowait will fail, above code reject to set async,
> >> then we will get a fault pfn and async=false.
> >>
> >> I guess this issue also exists in "QEMU write protected mapping" as
> >> you mentioned above.
> >
> > Yes, it does. As far as i understand, what that check does from a high
> > level pov is:
> >
> > - Did get_user_pages_nowait() fail due to a swapped out page (in which
> > case we should try to swappin the page asynchronously), or due to
> > another reason (for which case an error should be returned).
> >
> > Using vma->vm_flags VM_WRITE for that is trying to guess why
> > get_user_pages_nowait() failed, because it (gup_nowait return values)
> > does not provide sufficient information by itself.
> >
>
> That is exactly what i did in the first version. :)
>
> You can see it and the reason why it switched to the new way (readonly memslot)
> in the above website (the first message in thread).
Userspace can create multiple mappings for the same memory region, for
example via shared memory (shm_open), and have different protections for
the two (or more) regions. I had old patch doing this, its attached.
> > Can't that be fixed separately?
> >
> > Another issue which is also present with the mmap(PROT_READ) scheme is
> > interaction with reexecute_instruction. That is, unless i am mistaken,
> > reexecute_instruction can succeed (return true) on a region that is
> > write protected. This breaks the "write faults on read-only slots exit
> > to userspace via EXIT_MMIO" behaviour.
>
> Sorry, Why? After re-entry to the guest, it can not generate a correct MMIO?
reexecute_instruction validates presence of GPA by looking at registered
memslots. But if the access is a write, and userspace memory map is
read-only, reexecute_instruction should exit via MMIO.
That is, reexecute_instruction must validate GPA using registered
memslots AND additionaly userspace map permission, not only registered
memslot.
Index: qemu-kvm-gpage-cache/cpu-common.h
===================================================================
--- qemu-kvm-gpage-cache.orig/cpu-common.h
+++ qemu-kvm-gpage-cache/cpu-common.h
@@ -33,6 +33,7 @@ ram_addr_t qemu_ram_alloc(ram_addr_t);
void qemu_ram_free(ram_addr_t addr);
/* This should only be used for ram local to a device. */
void *qemu_get_ram_ptr(ram_addr_t addr);
+void *qemu_get_ram_ptr_guest(ram_addr_t addr);
/* This should not be used by devices. */
int do_qemu_ram_addr_from_host(void *ptr, ram_addr_t *ram_addr);
ram_addr_t qemu_ram_addr_from_host(void *ptr);
Index: qemu-kvm-gpage-cache/exec.c
===================================================================
--- qemu-kvm-gpage-cache.orig/exec.c
+++ qemu-kvm-gpage-cache/exec.c
@@ -35,6 +35,7 @@
#include "exec-all.h"
#include "qemu-common.h"
#include "cache-utils.h"
+#include "sysemu.h"
#if !defined(TARGET_IA64)
#include "tcg.h"
@@ -124,6 +125,7 @@ static int in_migration;
typedef struct RAMBlock {
uint8_t *host;
+ uint8_t *guest;
ram_addr_t offset;
ram_addr_t length;
struct RAMBlock *next;
@@ -2450,7 +2452,8 @@ static long gethugepagesize(const char *
return fs.f_bsize;
}
-static void *file_ram_alloc(ram_addr_t memory, const char *path)
+static void *file_ram_alloc(ram_addr_t memory, const char *path,
+ RAMBlock *block)
{
char *filename;
void *area;
@@ -2507,7 +2510,12 @@ static void *file_ram_alloc(ram_addr_t m
* MAP_PRIVATE is requested. For mem_prealloc we mmap as MAP_SHARED
* to sidestep this quirk.
*/
- flags = mem_prealloc ? MAP_POPULATE|MAP_SHARED : MAP_PRIVATE;
+ if (mem_guest_map)
+ flags = MAP_SHARED;
+ else if (mem_prealloc)
+ flags = MAP_POPULATE|MAP_SHARED;
+ else
+ flags = MAP_PRIVATE;
area = mmap(0, memory, PROT_READ|PROT_WRITE, flags, fd, 0);
#else
area = mmap(0, memory, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
@@ -2517,12 +2525,22 @@ static void *file_ram_alloc(ram_addr_t m
close(fd);
return (NULL);
}
+ if (mem_guest_map) {
+ block->guest = mmap(0, memory, PROT_READ|PROT_WRITE, flags, fd, 0);
+ if (block->guest == MAP_FAILED) {
+ perror("alloc_mem_area: can't mmap guest map");
+ munmap(area, memory);
+ close(fd);
+ return NULL;
+ }
+ }
return area;
}
#else
-static void *file_ram_alloc(ram_addr_t memory, const char *path)
+static void *file_ram_alloc(ram_addr_t memory, const char *path,
+ RAMBlock *block)
{
return NULL;
}
@@ -2538,7 +2556,7 @@ ram_addr_t qemu_ram_alloc(ram_addr_t siz
size = TARGET_PAGE_ALIGN(size);
new_block = qemu_malloc(sizeof(*new_block));
- new_block->host = file_ram_alloc(size, mem_path);
+ new_block->host = file_ram_alloc(size, mem_path, new_block);
if (!new_block->host) {
#if defined(TARGET_S390X) && defined(CONFIG_KVM)
/* XXX S390 KVM requires the topmost vma of the RAM to be < 256GB */
@@ -2584,7 +2602,8 @@ void qemu_ram_free(ram_addr_t addr)
It should not be used for general purpose DMA.
Use cpu_physical_memory_map/cpu_physical_memory_rw instead.
*/
-void *qemu_get_ram_ptr(ram_addr_t addr)
+
+static void *__qemu_get_ram_ptr(ram_addr_t addr)
{
RAMBlock *prev;
RAMBlock **prevp;
@@ -2610,9 +2629,27 @@ void *qemu_get_ram_ptr(ram_addr_t addr)
block->next = *prevp;
*prevp = block;
}
+ return block;
+}
+
+void *qemu_get_ram_ptr(ram_addr_t addr)
+{
+ RAMBlock *block = __qemu_get_ram_ptr(addr);
+
return block->host + (addr - block->offset);
}
+void *qemu_get_ram_ptr_guest(ram_addr_t addr)
+{
+ RAMBlock *block;
+
+ if (!mem_guest_map)
+ return qemu_get_ram_ptr(addr);
+
+ block = __qemu_get_ram_ptr(addr);
+ return block->guest + (addr - block->offset);
+}
+
int do_qemu_ram_addr_from_host(void *ptr, ram_addr_t *ram_addr)
{
RAMBlock *prev;
Index: qemu-kvm-gpage-cache/qemu-kvm.c
===================================================================
--- qemu-kvm-gpage-cache.orig/qemu-kvm.c
+++ qemu-kvm-gpage-cache/qemu-kvm.c
@@ -2327,7 +2327,7 @@ void kvm_set_phys_mem(target_phys_addr_t
#endif
r = kvm_register_phys_mem(kvm_context, start_addr,
- qemu_get_ram_ptr(phys_offset), size, 0);
+ qemu_get_ram_ptr_guest(phys_offset), size, 0);
if (r < 0) {
printf("kvm_cpu_register_physical_memory: failed\n");
exit(1);
Index: qemu-kvm-gpage-cache/sysemu.h
===================================================================
--- qemu-kvm-gpage-cache.orig/sysemu.h
+++ qemu-kvm-gpage-cache/sysemu.h
@@ -15,6 +15,7 @@
/* vl.c */
extern const char *bios_name;
+extern int mem_guest_map;
#define QEMU_FILE_TYPE_BIOS 0
#define QEMU_FILE_TYPE_KEYMAP 1
Index: qemu-kvm-gpage-cache/vl.c
===================================================================
--- qemu-kvm-gpage-cache.orig/vl.c
+++ qemu-kvm-gpage-cache/vl.c
@@ -248,6 +248,7 @@ const char *mem_path = NULL;
#ifdef MAP_POPULATE
int mem_prealloc = 1; /* force preallocation of physical target memory */
#endif
+int mem_guest_map = 1; /* separate qemu/guest mappings for RAM */
#ifdef TARGET_ARM
int old_param = 0;
#endif
