Re: [RESEND PATCH 7/7] hw/arm/virt: [DO NOT UPSTREAM] Enforce compatibility with older kernels

[Eric Auger <eric.auger@redhat.com>]

On 10/28/25 11:37 AM, Peter Maydell wrote:
> On Thu, 16 Oct 2025 at 15:01, Eric Auger <eric.auger@redhat.com> wrote:
>> This is an example on how to use the new CPU options. This catters to
>> distributions who want machines to be migratable (forward and backward)
>> accross different host kernel versions in case KVM registers exposed
>> to qemu vary accross kernels. This patch is not meant to be upstreamed
>> as it is really kernel dependent. The goal is to illustrate how this
>> would be used.
>>
>> In this example, For 10_1 machines types and older we apply the following
>> host kernel related compats:
>>
>> 1) Make sure the KVM_REG_ARM_VENDOR_HYP_BMAP_2 exposed from v6.15 onwards
>>    is ignored/hidden.
>> 2) Make sure TCR_EL1, PIRE0_EL1, PIR_EL1 are always seen by qemu
>>    although not exposed by KVM. They were unconditionnally exposed before
>>    v6.13 while from v6.13 they are only exposed if supported by the guest.
>>
>> This will allow 10_1 machines types and older machines to migrate
>> forward and backward from old downstream kernels that do not feature
>> those changes to newer kernels (>= v6.15).
>>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>> ---
>>  hw/arm/virt.c | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
>> index 175023897a..c4f9b82c38 100644
>> --- a/hw/arm/virt.c
>> +++ b/hw/arm/virt.c
>> @@ -97,6 +97,23 @@ static GlobalProperty arm_virt_compat[] = {
>>  };
>>  static const size_t arm_virt_compat_len = G_N_ELEMENTS(arm_virt_compat);
>>
>> +/*
>> + * if a 10_1 machine type or older is used:
>> + * 1) make sure TCR_EL1, PIRE0_EL1, PIR_EL1 are enforced, even if they are not
>> + *    exposed by the kernel
>> + * 2) hide KVM_REG_ARM_VENDOR_HYP_BMAP_2
>> + */
>> +static GlobalProperty arm_virt_kernel_compat_10_1[] = {
>> +    /* KVM_REG_ARM_VENDOR_HYP_BMAP_2 */
>> +    { TYPE_ARM_CPU, "kvm-hidden-regs", "0x6030000000160003" },
>> +    /* TCR_EL1, PIRE0_EL1, PIR_EL1 */
>> +    { TYPE_ARM_CPU, "kvm-enforced-regs",
>> +      "0x603000000013c103, 0x603000000013c512, 0x603000000013c513" },
> Strings which are lists of long hex numbers? Is there a
> more readable way to do this?

forgot to mention that we could use x- prefix for those properties to
enphasize they are rather experimental and not meant to be used commonly.

Eric
>
> -- PMM
>