Message-ID:
 <SN6PR02MB41571C1A6DE8756D72176E93D43AA@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 29 Aug 2025 17:03:13 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>
CC: "K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang
	<haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui
	<decui@...rosoft.com>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Nuno Das Neves
	<nunodasneves@...ux.microsoft.com>, Tianyu Lan <tiala@...rosoft.com>, Li Tian
	<litian@...hat.com>, Philipp Rudo <prudo@...hat.com>
Subject: RE: [PATCH v4] x86/hyperv: Fix kdump on Azure CVMs

From: Vitaly Kuznetsov <vkuznets@...hat.com> Sent: Thursday, August 28, 2025 2:16 AM
> 
> Azure CVM instance types featuring a paravisor hang upon kdump. The
> investigation shows that makedumpfile causes a hang when it steps on a page
> which was previously share with the host
> (HVCALL_MODIFY_SPARSE_GPA_PAGE_HOST_VISIBILITY). The new kernel has no
> knowledge of these 'special' regions (which are Vmbus connection pages,
> GPADL buffers, ...). There are several ways to approach the issue:
> - Convey the knowledge about these regions to the new kernel somehow.
> - Unshare these regions before accessing in the new kernel (it is unclear
> if there's a way to query the status for a given GPA range).
> - Unshare these regions before jumping to the new kernel (which this patch
> implements).
> 
> To make the procedure as robust as possible, store PFN ranges of shared
> regions in a linked list instead of storing GVAs and re-using
> hv_vtom_set_host_visibility(). This also allows to avoid memory allocation
> on the kdump/kexec path.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Looks good!

Adding Confidential Computing mailing list (linux-coco@...ts.linux.dev)
for visibility.

Reviewed-by: Michael Kelley <mhklinux@...look.com>

> ---
> Changes since v3 [Michael Kelley]:
>  - Employ x86_platform.guest.enc_kexec_{begin,finish} hooks.
>  - Don't use spinlock in what's now hv_vtom_kexec_finish().
>  - Handle possible hypercall failures in hv_mark_gpa_visibility()
>    symmetrically; change hv_list_enc_remove() to return -ENOMEM as well.
>  - Rebase to the latest hyperv/next.
> ---
>  arch/x86/hyperv/ivm.c | 211 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 210 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
> index ade6c665c97e..a4615b889f3e 100644
> --- a/arch/x86/hyperv/ivm.c
> +++ b/arch/x86/hyperv/ivm.c
> @@ -462,6 +462,195 @@ void hv_ivm_msr_read(u64 msr, u64 *value)
>  		hv_ghcb_msr_read(msr, value);
>  }
> 
> +/*
> + * Keep track of the PFN regions which were shared with the host. The access
> + * must be revoked upon kexec/kdump (see hv_ivm_clear_host_access()).
> + */
> +struct hv_enc_pfn_region {
> +	struct list_head list;
> +	u64 pfn;
> +	int count;
> +};
> +
> +static LIST_HEAD(hv_list_enc);
> +static DEFINE_RAW_SPINLOCK(hv_list_enc_lock);
> +
> +static int hv_list_enc_add(const u64 *pfn_list, int count)
> +{
> +	struct hv_enc_pfn_region *ent;
> +	unsigned long flags;
> +	u64 pfn;
> +	int i;
> +
> +	for (i = 0; i < count; i++) {
> +		pfn = pfn_list[i];
> +
> +		raw_spin_lock_irqsave(&hv_list_enc_lock, flags);
> +		/* Check if the PFN already exists in some region first */
> +		list_for_each_entry(ent, &hv_list_enc, list) {
> +			if ((ent->pfn <= pfn) && (ent->pfn + ent->count - 1 >= pfn))
> +				/* Nothing to do - pfn is already in the list */
> +				goto unlock_done;
> +		}
> +
> +		/*
> +		 * Check if the PFN is adjacent to an existing region. Growing
> +		 * a region can make it adjacent to another one but merging is
> +		 * not (yet) implemented for simplicity. A PFN cannot be added
> +		 * to two regions to keep the logic in hv_list_enc_remove()
> +		 * correct.
> +		 */
> +		list_for_each_entry(ent, &hv_list_enc, list) {
> +			if (ent->pfn + ent->count == pfn) {
> +				/* Grow existing region up */
> +				ent->count++;
> +				goto unlock_done;
> +			} else if (pfn + 1 == ent->pfn) {
> +				/* Grow existing region down */
> +				ent->pfn--;
> +				ent->count++;
> +				goto unlock_done;
> +			}
> +		}
> +		raw_spin_unlock_irqrestore(&hv_list_enc_lock, flags);
> +
> +		/* No adjacent region found -- create a new one */
> +		ent = kzalloc(sizeof(struct hv_enc_pfn_region), GFP_KERNEL);
> +		if (!ent)
> +			return -ENOMEM;
> +
> +		ent->pfn = pfn;
> +		ent->count = 1;
> +
> +		raw_spin_lock_irqsave(&hv_list_enc_lock, flags);
> +		list_add(&ent->list, &hv_list_enc);
> +
> +unlock_done:
> +		raw_spin_unlock_irqrestore(&hv_list_enc_lock, flags);
> +	}
> +
> +	return 0;
> +}
> +
> +static int hv_list_enc_remove(const u64 *pfn_list, int count)
> +{
> +	struct hv_enc_pfn_region *ent, *t;
> +	struct hv_enc_pfn_region new_region;
> +	unsigned long flags;
> +	u64 pfn;
> +	int i;
> +
> +	for (i = 0; i < count; i++) {
> +		pfn = pfn_list[i];
> +
> +		raw_spin_lock_irqsave(&hv_list_enc_lock, flags);
> +		list_for_each_entry_safe(ent, t, &hv_list_enc, list) {
> +			if (pfn == ent->pfn + ent->count - 1) {
> +				/* Removing tail pfn */
> +				ent->count--;
> +				if (!ent->count) {
> +					list_del(&ent->list);
> +					kfree(ent);
> +				}
> +				goto unlock_done;
> +			} else if (pfn == ent->pfn) {
> +				/* Removing head pfn */
> +				ent->count--;
> +				ent->pfn++;
> +				if (!ent->count) {
> +					list_del(&ent->list);
> +					kfree(ent);
> +				}
> +				goto unlock_done;
> +			} else if (pfn > ent->pfn && pfn < ent->pfn + ent->count - 1) {
> +				/*
> +				 * Removing a pfn in the middle. Cut off the tail
> +				 * of the existing region and create a template for
> +				 * the new one.
> +				 */
> +				new_region.pfn = pfn + 1;
> +				new_region.count = ent->count - (pfn - ent->pfn + 1);
> +				ent->count = pfn - ent->pfn;
> +				goto unlock_split;
> +			}
> +
> +		}
> +unlock_done:
> +		raw_spin_unlock_irqrestore(&hv_list_enc_lock, flags);
> +		continue;
> +
> +unlock_split:
> +		raw_spin_unlock_irqrestore(&hv_list_enc_lock, flags);
> +
> +		ent = kzalloc(sizeof(struct hv_enc_pfn_region), GFP_KERNEL);
> +		if (!ent)
> +			return -ENOMEM;
> +
> +		ent->pfn = new_region.pfn;
> +		ent->count = new_region.count;
> +
> +		raw_spin_lock_irqsave(&hv_list_enc_lock, flags);
> +		list_add(&ent->list, &hv_list_enc);
> +		raw_spin_unlock_irqrestore(&hv_list_enc_lock, flags);
> +	}
> +
> +	return 0;
> +}
> +
> +/* Stop new private<->shared conversions */
> +static void hv_vtom_kexec_begin(void)
> +{
> +	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
> +		return;
> +
> +	/*
> +	 * Crash kernel reaches here with interrupts disabled: can't wait for
> +	 * conversions to finish.
> +	 *
> +	 * If race happened, just report and proceed.
> +	 */
> +	if (!set_memory_enc_stop_conversion())
> +		pr_warn("Failed to stop shared<->private conversions\n");
> +}
> +
> +static void hv_vtom_kexec_finish(void)
> +{
> +	struct hv_gpa_range_for_visibility *input;
> +	struct hv_enc_pfn_region *ent;
> +	unsigned long flags;
> +	u64 hv_status;
> +	int cur, i;
> +
> +	local_irq_save(flags);
> +	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
> +
> +	if (unlikely(!input))
> +		goto out;
> +
> +	list_for_each_entry(ent, &hv_list_enc, list) {
> +		for (i = 0, cur = 0; i < ent->count; i++) {
> +			input->gpa_page_list[cur] = ent->pfn + i;
> +			cur++;
> +
> +			if (cur == HV_MAX_MODIFY_GPA_REP_COUNT || i == ent->count - 1) {
> +				input->partition_id = HV_PARTITION_ID_SELF;
> +				input->host_visibility = VMBUS_PAGE_NOT_VISIBLE;
> +				input->reserved0 = 0;
> +				input->reserved1 = 0;
> +				hv_status = hv_do_rep_hypercall(
> +					HVCALL_MODIFY_SPARSE_GPA_PAGE_HOST_VISIBILITY,
> +					cur, 0, input, NULL);
> +				WARN_ON_ONCE(!hv_result_success(hv_status));
> +				cur = 0;
> +			}
> +		}
> +
> +	}
> +
> +out:
> +	local_irq_restore(flags);
> +}
> +
>  /*
>   * hv_mark_gpa_visibility - Set pages visible to host via hvcall.
>   *
> @@ -475,6 +664,7 @@ static int hv_mark_gpa_visibility(u16 count, const u64 pfn[],
>  	struct hv_gpa_range_for_visibility *input;
>  	u64 hv_status;
>  	unsigned long flags;
> +	int ret;
> 
>  	/* no-op if partition isolation is not enabled */
>  	if (!hv_is_isolation_supported())
> @@ -486,6 +676,13 @@ static int hv_mark_gpa_visibility(u16 count, const u64 pfn[],
>  		return -EINVAL;
>  	}
> 
> +	if (visibility == VMBUS_PAGE_NOT_VISIBLE)
> +		ret = hv_list_enc_remove(pfn, count);
> +	else
> +		ret = hv_list_enc_add(pfn, count);
> +	if (ret)
> +		return ret;
> +
>  	local_irq_save(flags);
>  	input = *this_cpu_ptr(hyperv_pcpu_input_arg);
> 
> @@ -506,8 +703,18 @@ static int hv_mark_gpa_visibility(u16 count, const u64 pfn[],
> 
>  	if (hv_result_success(hv_status))
>  		return 0;
> +
> +	if (visibility == VMBUS_PAGE_NOT_VISIBLE)
> +		ret = hv_list_enc_add(pfn, count);
>  	else
> -		return -EFAULT;
> +		ret = hv_list_enc_remove(pfn, count);
> +	/*
> +	 * There's no good way to recover from -ENOMEM here, the accounting is
> +	 * wrong either way.
> +	 */
> +	WARN_ON_ONCE(ret);
> +
> +	return -EFAULT;
>  }
> 
>  /*
> @@ -669,6 +876,8 @@ void __init hv_vtom_init(void)
>  	x86_platform.guest.enc_tlb_flush_required = hv_vtom_tlb_flush_required;
>  	x86_platform.guest.enc_status_change_prepare = hv_vtom_clear_present;
>  	x86_platform.guest.enc_status_change_finish = hv_vtom_set_host_visibility;
> +	x86_platform.guest.enc_kexec_begin = hv_vtom_kexec_begin;
> +	x86_platform.guest.enc_kexec_finish = hv_vtom_kexec_finish;
> 
>  	/* Set WB as the default cache mode. */
>  	guest_force_mtrr_state(NULL, 0, MTRR_TYPE_WRBACK);
> --
> 2.50.1

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