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Re: [PATCH 0/2] expose host-phys-bits to guest
From: |
Claudio Fontana |
Subject: |
Re: [PATCH 0/2] expose host-phys-bits to guest |
Date: |
Thu, 1 Sep 2022 16:55:56 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 |
On 9/1/22 08:07, Xiaoyao Li wrote:
> On 8/31/2022 8:50 PM, Gerd Hoffmann wrote:
>> When the guest (firmware specifically) knows how big
>> the address space actually is it can be used better.
>>
>> Some more background:
>> https://bugzilla.redhat.com/show_bug.cgi?id=2084533
>
> QEMU enables host-phys-bits for "-cpu host/max" in
> host_cpu_max_instance_init();
No, in host_cpu_max_instance_init the default for host-phys-bits is set to on.
You can still get the phys bits adjusted if you set the property to on manually
for other cpu models.
>
> I think the problem is for all the named CPU model, that they don't have
> phys_bits defined. Thus they all have "cpu->phys-bits == 0", which leads
> to cpu->phys_bits = TCG_PHYS_ADDR_BITS (36 for 32-bits build and 40 for
> 64-bits build)
>
> Anyway, IMO, guest including guest firmware, should always consult from
> CPUID leaf 0x80000008 for physical address length. Tt is the duty of
> userspace VMM, here QEMU, to ensure VM's host physical address length
> not exceeding host's. If userspace VMM cannot ensure this, guest is
> likely hitting problem.
>
>> This is a RfC series exposes the information via cpuid.
>>
>> take care,
>> Gerd
>>
>> Gerd Hoffmann (2):
>> [hack] reserve bit KVM_HINTS_HOST_PHYS_BITS
>> [RfC] expose host-phys-bits to guest
>>
>> include/standard-headers/asm-x86/kvm_para.h | 3 ++-
>> target/i386/cpu.h | 3 ---
>> hw/i386/microvm.c | 6 +++++-
>> target/i386/cpu.c | 3 +--
>> target/i386/host-cpu.c | 4 +++-
>> target/i386/kvm/kvm.c | 1 +
>> 6 files changed, 12 insertions(+), 8 deletions(-)
>>
>
>
- Re: [PATCH 0/2] expose host-phys-bits to guest, Xiaoyao Li, 2022/09/01
- Re: [PATCH 0/2] expose host-phys-bits to guest, Gerd Hoffmann, 2022/09/01
- Re: [PATCH 0/2] expose host-phys-bits to guest, Xiaoyao Li, 2022/09/01
- Re: [PATCH 0/2] expose host-phys-bits to guest, Gerd Hoffmann, 2022/09/01
- Re: [PATCH 0/2] expose host-phys-bits to guest, Xiaoyao Li, 2022/09/01
- Re: [PATCH 0/2] expose host-phys-bits to guest, Gerd Hoffmann, 2022/09/02
- Re: [PATCH 0/2] expose host-phys-bits to guest, Michael S. Tsirkin, 2022/09/02
- Re: [PATCH 0/2] expose host-phys-bits to guest, Gerd Hoffmann, 2022/09/02
- Re: [PATCH 0/2] expose host-phys-bits to guest, Michael S. Tsirkin, 2022/09/04
- Re: [PATCH 0/2] expose host-phys-bits to guest, Gerd Hoffmann, 2022/09/05
Re: [PATCH 0/2] expose host-phys-bits to guest,
Claudio Fontana <=