On 08.09.23 17:13, Ani Sinha wrote:
On 08-Sep-2023, at 7:46 PM, David Hildenbrand <david@redhat.com> wrote:
On 08.09.23 16:12, Ani Sinha wrote:
On 08-Sep-2023, at 3:58 PM, David Hildenbrand <david@redhat.com> wrote:
On 08.09.23 11:50, Ani Sinha wrote:
Depending on the number of available address bits of the current processor, a
VM can only use a certain maximum amount of memory and no more. This change
makes sure that a VM is not configured to have more memory than what it can use
with the current processor settings when started. Additionally, the change adds
checks during memory hotplug to ensure that the VM does not end up getting more
memory than what it can actually use after hotplug.
Currently, both the above checks are only for pc (x86) platform.
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1235403
CC: imammedo@redhat.com
Signed-off-by: Ani Sinha <anisinha@redhat.com>
---
hw/i386/pc.c | 45 ++++++++++++++++++++++++++++++++++++++++++
hw/mem/memory-device.c | 6 ++++++
include/hw/boards.h | 9 +++++++++
3 files changed, 60 insertions(+)
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 54838c0c41..f84e4c4916 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -31,6 +31,7 @@
#include "hw/i386/topology.h"
#include "hw/i386/fw_cfg.h"
#include "hw/i386/vmport.h"
+#include "hw/mem/memory-device.h"
#include "sysemu/cpus.h"
#include "hw/block/fdc.h"
#include "hw/ide/internal.h"
@@ -1006,6 +1007,17 @@ void pc_memory_init(PCMachineState *pcms,
exit(EXIT_FAILURE);
}
+ /*
+ * check if the VM started with more ram configured than max physical
+ * address available with the current processor.
+ */
+ if (machine->ram_size > maxphysaddr + 1) {
+ error_report("Address space limit 0x%"PRIx64" < 0x%"PRIx64
+ " (max configured memory), phys-bits too low (%u)",
+ maxphysaddr, machine->ram_size, cpu->phys_bits);
+ exit(EXIT_FAILURE);
+ }
... I know that this used to be a problem in the past, but nowadays we already
do have similar checks in place?
$ ./build/qemu-system-x86_64 -m 4T -machine q35,memory-backend=mem0 -object
memory-backend-ram,id=mem0,size=4T,reserve=off
qemu-system-x86_64: Address space limit 0xffffffffff < 0x5077fffffff phys-bits
too low (40)
So you are saying that this is OK and should be allowed? On a 32 bit processor
that can access only 4G memory, I am spinning up a 10G VM.
Would that 32bit process have PAE (Physical Address Extension) and still be
able to access that memory?
You are sidestepping my point. Sure, we can improve the condition check by
checking for PAE CPUID etc but that is not the issue I am trying too point out.
What if the processor did not have PAE? Would we allow a VM to have memory size
which the processor can’t access? There is no such check today it would seem.
Indeed, because the implementation for 32bit in pc_max_used_gpa() is wrong.
Note that for 64bit it does the right thing, even with memory hotplug,
because the PCI64 hole is placed above the memory device region.
So I think we should tackle that via pc_max_used_gpa().
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 54838c0c41..d187890675 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -908,9 +908,12 @@ static hwaddr pc_max_used_gpa(PCMachineState *pcms,
uint64_t pci_hole64_size)
{
X86CPU *cpu = X86_CPU(first_cpu);
- /* 32-bit systems don't have hole64 thus return max CPU address */
- if (cpu->phys_bits <= 32) {
- return ((hwaddr)1 << cpu->phys_bits) - 1;
+ /*
+ * 32-bit systems don't have hole64, but we might have a region for
+ * memory hotplug.
+ */
+ if (!(cpu->env.features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM)) {
+ return pc_pci_hole64_start() - 1;
}
return pc_pci_hole64_start() + pci_hole64_size - 1;
That implies:
./build/qemu-system-x86_64 -cpu pentium -m size=4G -nodefaults -nographic
qemu-system-x86_64: Address space limit 0xffffffff < 0x13fffffff
phys-bits too low (32)
As we have memory over 4G (due to PCI hole), that would now correctly fail.
However, what works is:
./build/qemu-system-x86_64 -cpu pentium -m size=3G -nodefaults -nographic
Weirdly enough, when setting cpu->phys_bits, we take care of PSE36 and
allow for 36bits in the address space.
So what works:
./build/qemu-system-x86_64 -cpu pentium,pse36=on -m size=32G -nodefaults
-nographic
And what doesn't:
./build/qemu-system-x86_64 -cpu pentium,pse36=on -m size=64G
-nodefaults -nographic -S
qemu-system-x86_64: Address space limit 0xfffffffff < 0x103fffffff
phys-bits too low (36)
However, we don't seem to have such handling in place for PAE (do we
have to extend that handling in x86_cpu_realizefn()?). Maybe pae should
always imply pse36, not sure ...