From: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx> Sent: Thursday, March 18, 2021 1:14 PM
>
> Linux has support for free page reporting now (36e66c554b5c) for
> virtualized environment. On Hyper-V when virtually backed VMs are
> configured, Hyper-V will advertise cold memory discard capability,
> when supported. This patch adds the support to hook into the free
> page reporting infrastructure and leverage the Hyper-V cold memory
> discard hint hypercall to report/free these pages back to the host.
>
> Signed-off-by: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>
> Tested-by: Matheus Castello <matheus@xxxxxxxxxxxxxxx>
> ---
> In V2:
> - Addressed feedback comments
> - Added page reporting config option tied to hyper-v balloon config
>
> In V3:
> - Addressed feedback from Vitaly
>
> In V4:
> - Queried and cached the Hyper-V extended capability for the lifetime
> of the VM
> - Addressed feedback from Michael Kelley.
> ---
> arch/x86/hyperv/hv_init.c | 45 +++++++++++++++-
> arch/x86/kernel/cpu/mshyperv.c | 9 ++--
> drivers/hv/Kconfig | 1 +
> drivers/hv/hv_balloon.c | 89 +++++++++++++++++++++++++++++++
> include/asm-generic/hyperv-tlfs.h | 35 +++++++++++-
> include/asm-generic/mshyperv.h | 3 +-
> 6 files changed, 174 insertions(+), 8 deletions(-)
>
[snip]
> +/* Bit mask of the extended capability to query: see HV_EXT_CAPABILITY_xxx */
> +bool hv_query_ext_cap(u64 cap_query)
> +{
> + /*
> + * The address of the 'hv_extended_cap' variable will be used as an
> + * output parameter to the hypercall below and so it should be
> + * compatible with 'virt_to_phys'. Which means, it's address should be
> + * directly mapped. Use 'static' to keep it compatible; stack variables
> + * can be virtually mapped, making them imcompatible with
> + * 'virt_to_phys'.
> + * Hypercall input/output addresses should also be 8-byte aligned.
> + */
> + static u64 hv_extended_cap __aligned(8);
> + static bool hv_extended_cap_queried;
> + u64 status;
> +
> + /*
> + * Querying extended capabilities is an extended hypercall. Check if the
> + * partition supports extended hypercall, first.
> + */
> + if (!(ms_hyperv.priv_high & HV_ENABLE_EXTENDED_HYPERCALLS))
> + return false;
> +
> + /* Extended capabilities do not change at runtime. */
> + if (hv_extended_cap_queried)
> + return hv_extended_cap & cap_query;
> +
> + status = hv_do_hypercall(HV_EXT_CALL_QUERY_CAPABILITIES, NULL,
> + &hv_extended_cap);
> + hv_extended_cap_queried = true;
What's the strategy for this flag in the unlikely event that the hypercall fails?
It doesn't seem right to have hv_query_ext_cap() fail, but leave the
static flag set to true. Just move that line down to after the status check
has succeeded?
> + status &= HV_HYPERCALL_RESULT_MASK;
> + if (status != HV_STATUS_SUCCESS) {
> + pr_err("Hyper-V: Extended query capabilities hypercall failed 0x%llx\n",
> + status);
> + return false;
> + }
> +
> + return hv_extended_cap & cap_query;
> +}
> +EXPORT_SYMBOL_GPL(hv_query_ext_cap);
Other than the above about the flag when the hypercall fails,
everything else looks good.
Michael
