Re: [PATCH v2 02/15] Drivers: hv: Move hv_vp_assist_page to common files

Naman Jain
2026-04-29T09:55:25Z

On 4/27/2026 11:07 AM, Michael Kelley wrote:
> From: Naman Jain <namjain@linux.microsoft.com> Sent: Thursday, April 23, 2026 5:42 AM
>>
>> Move the logic to initialize and export hv_vp_assist_page from x86
>> architecture code to Hyper-V common code to allow it to be used for
>> upcoming arm64 support in MSHV_VTL driver.
>> Note: This change also improves error handling - if VP assist page
>> allocation fails, hyperv_init() now returns early instead of
>> continuing with partial initialization.
>>
>> Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
>> Reviewed-by: Roman Kisel <vdso@mailbox.org>
>> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
>> ---
>> arch/x86/hyperv/hv_init.c | 88 +-----------------------------
>> arch/x86/include/asm/mshyperv.h | 14 -----
>> drivers/hv/hv_common.c | 94 ++++++++++++++++++++++++++++++++-
>> include/asm-generic/mshyperv.h | 16 ++++++
>> include/hyperv/hvgdk_mini.h | 6 ++-
>> 5 files changed, 115 insertions(+), 103 deletions(-)
>>
>> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
>> index 323adc93f2dc..75a98b5e451b 100644
>> --- a/arch/x86/hyperv/hv_init.c
>> +++ b/arch/x86/hyperv/hv_init.c
>> @@ -81,9 +81,6 @@ union hv_ghcb * __percpu *hv_ghcb_pg;
>> /* Storage to save the hypercall page temporarily for hibernation */
>> static void *hv_hypercall_pg_saved;
>>
>> -struct hv_vp_assist_page **hv_vp_assist_page;
>> -EXPORT_SYMBOL_GPL(hv_vp_assist_page);
>> -
>> static int hyperv_init_ghcb(void)
>> {
>> u64 ghcb_gpa;
>> @@ -117,59 +114,12 @@ static int hyperv_init_ghcb(void)
>>
>> static int hv_cpu_init(unsigned int cpu)
>> {
>> - union hv_vp_assist_msr_contents msr = { 0 };
>> - struct hv_vp_assist_page **hvp;
>> int ret;
>>
>> ret = hv_common_cpu_init(cpu);
>> if (ret)
>> return ret;
>>
>> - if (!hv_vp_assist_page)
>> - return 0;
>> -
>> - hvp = &hv_vp_assist_page[cpu];
>> - if (hv_root_partition()) {
>> - /*
>> - * For root partition we get the hypervisor provided VP assist
>> - * page, instead of allocating a new page.
>> - */
>> - rdmsrq(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> - *hvp = memremap(msr.pfn << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT,
>> - PAGE_SIZE, MEMREMAP_WB);
>> - } else {
>> - /*
>> - * The VP assist page is an "overlay" page (see Hyper-V TLFS's
>> - * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
>> - * out to make sure we always write the EOI MSR in
>> - * hv_apic_eoi_write() *after* the EOI optimization is disabled
>> - * in hv_cpu_die(), otherwise a CPU may not be stopped in the
>> - * case of CPU offlining and the VM will hang.
>> - */
>> - if (!*hvp) {
>> - *hvp = __vmalloc(PAGE_SIZE, GFP_KERNEL | __GFP_ZERO);
>> -
>> - /*
>> - * Hyper-V should never specify a VM that is a Confidential
>> - * VM and also running in the root partition. Root partition
>> - * is blocked to run in Confidential VM. So only decrypt assist
>> - * page in non-root partition here.
>> - */
>> - if (*hvp && !ms_hyperv.paravisor_present && hv_isolation_type_snp()) {
>> - WARN_ON_ONCE(set_memory_decrypted((unsigned long)(*hvp), 1));
>> - memset(*hvp, 0, PAGE_SIZE);
>> - }
>> - }
>> -
>> - if (*hvp)
>> - msr.pfn = vmalloc_to_pfn(*hvp);
>> -
>> - }
>> - if (!WARN_ON(!(*hvp))) {
>> - msr.enable = 1;
>> - wrmsrq(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> - }
>> -
>> /* Allow Hyper-V stimer vector to be injected from Hypervisor. */
>> if (ms_hyperv.misc_features & HV_STIMER_DIRECT_MODE_AVAILABLE)
>> apic_update_vector(cpu, HYPERV_STIMER0_VECTOR, true);
>> @@ -286,23 +236,6 @@ static int hv_cpu_die(unsigned int cpu)
>>
>> hv_common_cpu_die(cpu);
>>
>> - if (hv_vp_assist_page && hv_vp_assist_page[cpu]) {
>> - union hv_vp_assist_msr_contents msr = { 0 };
>> - if (hv_root_partition()) {
>> - /*
>> - * For root partition the VP assist page is mapped to
>> - * hypervisor provided page, and thus we unmap the
>> - * page here and nullify it, so that in future we have
>> - * correct page address mapped in hv_cpu_init.
>> - */
>> - memunmap(hv_vp_assist_page[cpu]);
>> - hv_vp_assist_page[cpu] = NULL;
>> - rdmsrq(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> - msr.enable = 0;
>> - }
>> - wrmsrq(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> - }
>> -
>> if (hv_reenlightenment_cb == NULL)
>> return 0;
>>
>> @@ -460,21 +393,6 @@ void __init hyperv_init(void)
>> if (hv_common_init())
>> return;
>>
>> - /*
>> - * The VP assist page is useless to a TDX guest: the only use we
>> - * would have for it is lazy EOI, which can not be used with TDX.
>> - */
>> - if (hv_isolation_type_tdx())
>> - hv_vp_assist_page = NULL;
>> - else
>> - hv_vp_assist_page = kzalloc_objs(*hv_vp_assist_page, nr_cpu_ids);
>> - if (!hv_vp_assist_page) {
>> - ms_hyperv.hints &= ~HV_X64_ENLIGHTENED_VMCS_RECOMMENDED;
>> -
>> - if (!hv_isolation_type_tdx())
>> - goto common_free;
>> - }
>> -
>> if (ms_hyperv.paravisor_present && hv_isolation_type_snp()) {
>> /* Negotiate GHCB Version. */
>> if (!hv_ghcb_negotiate_protocol())
>> @@ -483,7 +401,7 @@ void __init hyperv_init(void)
>>
>> hv_ghcb_pg = alloc_percpu(union hv_ghcb *);
>> if (!hv_ghcb_pg)
>> - goto free_vp_assist_page;
>> + goto free_ghcb_page;
>
> Seems like this should be "goto common_free". The allocation of
> hv_ghcb_pg has failed, so going to a label where hv_ghcb_pg is
> freed seems redundant. It works since free_percpu() checks for
> a NULL argument, but it's a bit unexpected since the common_free
> label is already there.

Thanks for catching this, I'll fix it.

>
>> }
>>
>> cpuhp = cpuhp_setup_state(CPUHP_AP_HYPERV_ONLINE, "x86/hyperv_init:online",
>> @@ -613,10 +531,6 @@ void __init hyperv_init(void)
>> cpuhp_remove_state(CPUHP_AP_HYPERV_ONLINE);
>> free_ghcb_page:
>> free_percpu(hv_ghcb_pg);
>> -free_vp_assist_page:
>> - kfree(hv_vp_assist_page);
>> - hv_vp_assist_page = NULL;
>> -common_free:
>> hv_common_free();
>> }
>>
>> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
>> index f64393e853ee..95b452387969 100644
>> --- a/arch/x86/include/asm/mshyperv.h
>> +++ b/arch/x86/include/asm/mshyperv.h
>> @@ -155,16 +155,6 @@ static inline u64 hv_do_fast_hypercall16(u16 code, u64 input1, u64 input2)
>> return _hv_do_fast_hypercall16(control, input1, input2);
>> }
>>
>> -extern struct hv_vp_assist_page **hv_vp_assist_page;
>> -
>> -static inline struct hv_vp_assist_page *hv_get_vp_assist_page(unsigned int cpu)
>> -{
>> - if (!hv_vp_assist_page)
>> - return NULL;
>> -
>> - return hv_vp_assist_page[cpu];
>> -}
>> -
>> void __init hyperv_init(void);
>> void hyperv_setup_mmu_ops(void);
>> void set_hv_tscchange_cb(void (*cb)(void));
>> @@ -254,10 +244,6 @@ static inline void hyperv_setup_mmu_ops(void) {}
>> static inline void set_hv_tscchange_cb(void (*cb)(void)) {}
>> static inline void clear_hv_tscchange_cb(void) {}
>> static inline void hyperv_stop_tsc_emulation(void) {};
>> -static inline struct hv_vp_assist_page *hv_get_vp_assist_page(unsigned int cpu)
>> -{
>> - return NULL;
>> -}
>> static inline int hyperv_flush_guest_mapping(u64 as) { return -1; }
>> static inline int hyperv_flush_guest_mapping_range(u64 as,
>> hyperv_fill_flush_list_func fill_func, void *data)
>> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
>> index 6b67ac616789..e8633bc51d56 100644
>> --- a/drivers/hv/hv_common.c
>> +++ b/drivers/hv/hv_common.c
>> @@ -28,7 +28,11 @@
>> #include <linux/slab.h>
>> #include <linux/dma-map-ops.h>
>> #include <linux/set_memory.h>
>> +#include <linux/vmalloc.h>
>> +#include <linux/io.h>
>> +#include <linux/hyperv.h>
>> #include <hyperv/hvhdk.h>
>> +#include <hyperv/hvgdk.h>
>> #include <asm/mshyperv.h>
>>
>> u64 hv_current_partition_id = HV_PARTITION_ID_SELF;
>> @@ -78,6 +82,8 @@ static struct ctl_table_header *hv_ctl_table_hdr;
>> u8 * __percpu *hv_synic_eventring_tail;
>> EXPORT_SYMBOL_GPL(hv_synic_eventring_tail);
>>
>> +struct hv_vp_assist_page **hv_vp_assist_page;
>> +EXPORT_SYMBOL_GPL(hv_vp_assist_page);
>> /*
>> * Hyper-V specific initialization and shutdown code that is
>> * common across all architectures. Called from architecture
>> @@ -92,6 +98,9 @@ void __init hv_common_free(void)
>> if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE)
>> hv_kmsg_dump_unregister();
>>
>> + kfree(hv_vp_assist_page);
>> + hv_vp_assist_page = NULL;
>> +
>> kfree(hv_vp_index);
>> hv_vp_index = NULL;
>>
>> @@ -394,6 +403,23 @@ int __init hv_common_init(void)
>> for (i = 0; i < nr_cpu_ids; i++)
>> hv_vp_index[i] = VP_INVAL;
>>
>> + /*
>> + * The VP assist page is useless to a TDX guest: the only use we
>> + * would have for it is lazy EOI, which can not be used with TDX.
>> + */
>> + if (hv_isolation_type_tdx()) {
>> + hv_vp_assist_page = NULL;
>> +#ifdef CONFIG_X86_64
>> + ms_hyperv.hints &= ~HV_X64_ENLIGHTENED_VMCS_RECOMMENDED;
>> +#endif
>
> I realize that this #ifdef went away for the reason I flagged in v1 of
> this patch set, but it's back again for a different reason.
>
> Let me suggest another approach. hv_common_init() is called from
> both the x86/64 and arm64 hyperv_init() functions. Immediately after
> the call to hv_common_init() in the x86/64 hyperv_init(), test
> hv_vp_assist_page for NULL and clear
> HV_X64_ENLIGHTENED_VMCS_RECOMMENDED if it is. No #ifdef is
> needed, and x86/64 specific hackery stays under arch/x86 instead of
> being in common code.

Acked. Thanks.

>
>> + } else {
>> + hv_vp_assist_page = kzalloc_objs(*hv_vp_assist_page, nr_cpu_ids);
>> + if (!hv_vp_assist_page) {
>> + hv_common_free();
>> + return -ENOMEM;
>> + }
>> + }
>> +
>> return 0;
>> }
>>
>> @@ -471,6 +497,8 @@ void __init ms_hyperv_late_init(void)
>>
>> int hv_common_cpu_init(unsigned int cpu)
>> {
>> + union hv_vp_assist_msr_contents msr = { 0 };
>> + struct hv_vp_assist_page **hvp;
>> void **inputarg, **outputarg;
>> u8 **synic_eventring_tail;
>> u64 msr_vp_index;
>> @@ -539,7 +567,53 @@ int hv_common_cpu_init(unsigned int cpu)
>> sizeof(u8), flags);
>> /* No need to unwind any of the above on failure here */
>> if (unlikely(!*synic_eventring_tail))
>> - ret = -ENOMEM;
>> + return -ENOMEM;
>> + }
>> +
>> + if (!hv_vp_assist_page)
>> + return ret;
>> +
>> + hvp = &hv_vp_assist_page[cpu];
>> + if (hv_root_partition()) {
>> + /*
>> + * For root partition we get the hypervisor provided VP assist
>> + * page, instead of allocating a new page.
>> + */
>> + msr.as_uint64 = hv_get_msr(HV_MSR_VP_ASSIST_PAGE);
>> + *hvp = memremap(msr.pfn << HV_VP_ASSIST_PAGE_ADDRESS_SHIFT,
>> + HV_HYP_PAGE_SIZE, MEMREMAP_WB);
>> + } else {
>> + /*
>> + * The VP assist page is an "overlay" page (see Hyper-V TLFS's
>> + * Section 5.2.1 "GPA Overlay Pages"). Here it must be zeroed
>> + * out to make sure that on x86/x64, we always write the EOI MSR in
>> + * hv_apic_eoi_write() *after* the EOI optimization is disabled
>> + * in hv_cpu_die(), otherwise a CPU may not be stopped in the
>> + * case of CPU offlining and the VM will hang.
>> + */
>> + if (!*hvp) {
>> + *hvp = __vmalloc(HV_HYP_PAGE_SIZE, flags | __GFP_ZERO);
>> +
>> + /*
>> + * Hyper-V should never specify a VM that is a Confidential
>> + * VM and also running in the root partition. Root partition
>> + * is blocked to run in Confidential VM. So only decrypt assist
>> + * page in non-root partition here.
>> + */
>> + if (*hvp &&
>> + !ms_hyperv.paravisor_present &&
>> + hv_isolation_type_snp()) {
>> + WARN_ON_ONCE(set_memory_decrypted((unsigned long)(*hvp), 1));
>> + memset(*hvp, 0, HV_HYP_PAGE_SIZE);
>> + }
>> + }
>> +
>> + if (*hvp)
>> + msr.pfn = page_to_hvpfn(vmalloc_to_page(*hvp));
>
> Your Patch 0 changelog mentions adding a comment about vmalloc_to_pfn(), which
> I didn't see anywhere. I'm not sure what that comment would say, so maybe it
> became unnecessary.

I think I mixed up two things. Changelog was about your suggestion to
add "x86/x64" in above comment about GPA Overlay Pages.
I also changed this function to page_to_hvpfn(vmalloc_to_page(*hvp)) as
per your suggestion.
Apologies for the confusion.

>
>> + }
>> + if (!WARN_ON(!(*hvp))) {
>> + msr.enable = 1;
>> + hv_set_msr(HV_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> }
>>
>> return ret;
>> @@ -566,6 +640,24 @@ int hv_common_cpu_die(unsigned int cpu)
>> *synic_eventring_tail = NULL;
>> }
>>
>> + if (hv_vp_assist_page && hv_vp_assist_page[cpu]) {
>> + union hv_vp_assist_msr_contents msr = { 0 };
>> +
>> + if (hv_root_partition()) {
>> + /*
>> + * For root partition the VP assist page is mapped to
>> + * hypervisor provided page, and thus we unmap the
>> + * page here and nullify it, so that in future we have
>> + * correct page address mapped in hv_cpu_init.
>> + */
>> + memunmap(hv_vp_assist_page[cpu]);
>> + hv_vp_assist_page[cpu] = NULL;
>> + msr.as_uint64 = hv_get_msr(HV_MSR_VP_ASSIST_PAGE);
>> + msr.enable = 0;
>> + }
>> + hv_set_msr(HV_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>> + }
>> +
>> return 0;
>> }
>>
>> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
>> index d37b68238c97..2810aa05dc73 100644
>> --- a/include/asm-generic/mshyperv.h
>> +++ b/include/asm-generic/mshyperv.h
>> @@ -25,6 +25,7 @@
>> #include <linux/nmi.h>
>> #include <asm/ptrace.h>
>> #include <hyperv/hvhdk.h>
>> +#include <hyperv/hvgdk.h>
>>
>> #define VTPM_BASE_ADDRESS 0xfed40000
>>
>> @@ -299,6 +300,16 @@ do { \
>> #define hv_status_debug(status, fmt, ...) \
>> hv_status_printk(debug, status, fmt, ##__VA_ARGS__)
>>
>> +extern struct hv_vp_assist_page **hv_vp_assist_page;
>> +
>> +static inline struct hv_vp_assist_page *hv_get_vp_assist_page(unsigned int cpu)
>> +{
>> + if (!hv_vp_assist_page)
>> + return NULL;
>> +
>> + return hv_vp_assist_page[cpu];
>> +}
>> +
>> const char *hv_result_to_string(u64 hv_status);
>> int hv_result_to_errno(u64 status);
>> void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die);
>> @@ -327,6 +338,11 @@ static inline enum hv_isolation_type hv_get_isolation_type(void)
>> {
>> return HV_ISOLATION_TYPE_NONE;
>> }
>> +
>> +static inline struct hv_vp_assist_page *hv_get_vp_assist_page(unsigned int cpu)
>> +{
>> + return NULL;
>> +}
>> #endif /* CONFIG_HYPERV */
>>
>> #if IS_ENABLED(CONFIG_MSHV_ROOT)
>> diff --git a/include/hyperv/hvgdk_mini.h b/include/hyperv/hvgdk_mini.h
>> index 056ef7b6b360..c72d04cd5ae4 100644
>> --- a/include/hyperv/hvgdk_mini.h
>> +++ b/include/hyperv/hvgdk_mini.h
>> @@ -149,6 +149,7 @@ struct hv_u128 {
>> #define HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT 12
>
> Can this X64 specific definition of the shift be eliminated entirely,
> and a single common definition for x86/64 and arm64 be used?
> As I understand it, the MSR layout is the same on both architectures.
> The one gotcha is that kvm_hv_set_msr() would need to be updated.
>
> HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_MASK defined below isn't
> used anywhere, so it could go away too. (The KVM selftest usage has
> its own definition.)
>
> I realize these are changes to a source code file that is derived from
> Windows, and I'm not sure of the guidelines for such changes. So maybe
> these suggestions have to be ignored ....

The VP assist page definition is common to both x86 and arm64, so the
address mask and shift can be shared. Also, I don't see the shift and
mask definitions in the Hyper-V header so seem to be specific to in
kernel usage.

>
>> #define HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_MASK \
>> (~((1ull << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) - 1))
>> +#define HV_MSR_VP_ASSIST_PAGE (HV_X64_MSR_VP_ASSIST_PAGE)
>
> This is the correct file for this #define, but it should be placed down around
> line 1148 or so with the other HV_MSR_* definitions in terms of HV_X64_MSR_*
>

Acked.

>>
>> /* Hyper-V Enlightened VMCS version mask in nested features CPUID */
>> #define HV_X64_ENLIGHTENED_VMCS_VERSION 0xff
>> @@ -410,6 +411,7 @@ union hv_x64_msr_hypercall_contents {
>> #if defined(CONFIG_ARM64)
>> #define HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE BIT(8)
>> #define HV_STIMER_DIRECT_MODE_AVAILABLE BIT(13)
>> +#define HV_VP_ASSIST_PAGE_ADDRESS_SHIFT 12
>> #endif /* CONFIG_ARM64 */
>>
>> #if defined(CONFIG_X86)
>> @@ -1163,6 +1165,8 @@ enum hv_register_name {
>> #define HV_MSR_STIMER0_CONFIG (HV_X64_MSR_STIMER0_CONFIG)
>> #define HV_MSR_STIMER0_COUNT (HV_X64_MSR_STIMER0_COUNT)
>>
>> +#define HV_VP_ASSIST_PAGE_ADDRESS_SHIFT HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT
>> +
>> #elif defined(CONFIG_ARM64) /* CONFIG_X86 */
>>
>> #define HV_MSR_CRASH_P0 (HV_REGISTER_GUEST_CRASH_P0)
>> @@ -1185,7 +1189,7 @@ enum hv_register_name {
>>
>> #define HV_MSR_STIMER0_CONFIG (HV_REGISTER_STIMER0_CONFIG)
>> #define HV_MSR_STIMER0_COUNT (HV_REGISTER_STIMER0_COUNT)
>> -
>> +#define HV_MSR_VP_ASSIST_PAGE (HV_REGISTER_VP_ASSIST_PAGE)
>
> Nit: This definition is slightly mis-aligned. It has spaces where there
> should be a tab to match the similar definitions above it.
>

Acked.

>> #endif /* CONFIG_ARM64 */
>>
>> union hv_explicit_suspend_register {
>> --
>> 2.43.0
>>

Regards,
Naman