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

From: Xiaoyao Li <xiaoyao.li@intel.com>
To: 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 14:07:11 +0800	[thread overview]
Message-ID: <957f0cc5-6887-3861-2b80-69a8c7cdd098@intel.com> (raw)
In-Reply-To: <20220831125059.170032-1-kraxel@redhat.com>

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();

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(-)
> 