On Wed, 04 May 2022 20:58:42 +0100,
Oliver Upton <oupton@xxxxxxxxxx> wrote:
>
> Hi Raghavendra,
>
> On Wed, May 04, 2022 at 06:44:15PM +0000, Raghavendra Rao Ananta wrote:
> > The hypercalls test currently defines its own *_BMAP_BIT_MAX macros to
> > define the last valid feature bit for each bitmap firmware register.
> > However, since these definitions are already present in the uapi header,
> > kvm.h, as *_BMAP_BIT_COUNT, and would help to keep the test updated as
> > features grow, use these instead.
>
> LOL, looks like I lost that one in the end! Still, the fact that you're
> patching the selftest highlights the fact that there is a nonzero chance
> of userspace using this value incorrectly expecting it to hold true
> across all kernels.
>
> Since this is the route going forward can we please consider documenting
> the fact that _BIT_COUNT *will* change and is not stable between kernel
> versions. Bad UAPI expectations could throw a wrench into this entire
> plan we've hatched for preserving hypercall ABI.
>
> Just a warning at the end of the register documentation would suffice.
Maybe something in the kvm.h file as well, as the includes are often
distributed without the kernel documentation. Something like:
diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index e523bb6eac67..3cde9f958eee 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -342,6 +342,10 @@ struct kvm_arm_copy_mte_tags {
enum {
KVM_REG_ARM_STD_BIT_TRNG_V1_0 = 0,
+ /*
+ * KVM_REG_ARM_STD_BMAP_BIT_COUNT will vary as new services
+ * are added, and is explicitely not part of the ABI.
+ */
KVM_REG_ARM_STD_BMAP_BIT_COUNT,
};
@@ -349,6 +353,10 @@ enum {
enum {
KVM_REG_ARM_STD_HYP_BIT_PV_TIME = 0,
+ /*
+ * KVM_REG_ARM_STD_HYP_BMAP_BIT_COUNT will vary as new
+ * services are added, and is explicitely not part of the ABI.
+ */
KVM_REG_ARM_STD_HYP_BMAP_BIT_COUNT,
};
@@ -357,6 +365,10 @@ enum {
enum {
KVM_REG_ARM_VENDOR_HYP_BIT_FUNC_FEAT = 0,
KVM_REG_ARM_VENDOR_HYP_BIT_PTP = 1,
+ /*
+ * KVM_REG_ARM_VENDOR_HYP_BMAP_BIT_COUNT will vary as new
+ * services are added, and is explicitely not part of the ABI.
+ */
KVM_REG_ARM_VENDOR_HYP_BMAP_BIT_COUNT,
};
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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