Re: [PATCH 0/2] expose host-phys-bits to guest

From: Claudio Fontana <cfontana@suse.de>
To: Xiaoyao Li <xiaoyao.li@intel.com>,
	Gerd Hoffmann <kraxel@redhat.com>,
	qemu-devel@nongnu.org
Cc: kvm@vger.kernel.org, Marcelo Tosatti <mtosatti@redhat.com>,
	Marcel Apfelbaum <marcel.apfelbaum@gmail.com>,
	Eduardo Habkost <eduardo@habkost.net>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Sergio Lopez <slp@redhat.com>
Subject: Re: [PATCH 0/2] expose host-phys-bits to guest
Date: Thu, 1 Sep 2022 16:55:56 +0200	[thread overview]
Message-ID: <cd14d1d5-3484-f1db-9473-9db7929789f3@suse.de> (raw)
In-Reply-To: <957f0cc5-6887-3861-2b80-69a8c7cdd098@intel.com>

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(-)
>>
> 
> 