Add support for redirected VTL0 interrupts #80

ricardon · 2025-05-22T16:45:53Z

Hi,

This pull request implements microsoft/openvmm#406. The issue describes the problem statement and proposed solution.

This code enables the changes made here: microsoft/openvmm#384.

Thank you for your consideration.

ricardon · 2025-05-22T16:58:33Z

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ricardon · 2025-05-22T17:07:54Z

@microsoft-github-policy-service agree [company="Intel Corporation"]

ricardon · 2025-05-22T17:08:26Z

@microsoft-github-policy-service agree company="Intel Corporation"

romank-msft · 2025-05-29T18:53:49Z

include/uapi/linux/mshv.h

 /* For x86-64 TDX only */
 #define MSHV_VTL_TDCALL _IOWR(MSHV_IOCTL, 0x32, struct mshv_tdcall)
 #define MSHV_VTL_READ_VMX_CR4_FIXED1 _IOR(MSHV_IOCTL, 0x33, __u64)
+#define MSHV_VTL_MAP_REDIRECTED_DEVICE_INTERRUPT _IOWR(MSHV_IOCTL, 0x38, \


that was taken recently by the KICK_CPUS ioctl. If you've rebased, please change to a free code.

Sure. I didn't realize. I will update it.

romank-msft · 2025-05-29T18:57:07Z

drivers/hv/mshv_vtl_main.c

+		 * vector is mapped.
+		 */
+		intr_data.vector = ret;
+		ret = copy_to_user(user_arg, &intr_data, sizeof(intr_data));


This might fail returning the number of bytes that weren't copied. Is that good to return from this ioctl?

Right. This does not look correct. I'll fix the error handling.

romank-msft · 2025-05-29T18:58:29Z

drivers/hv/mshv_vtl_main.c

+		return (long)-EFAULT;
+
+	if (!intr_data.create_mapping)
+		/* User space provides the hardware vector to unmap. */


maybe move the comment above the if statement?

Sure. Will do.

romank-msft · 2025-05-29T19:00:10Z

drivers/hv/mshv_vtl_main.c


+static void do_assert_single_proxy_intr(const u32 vector, struct mshv_vtl_run *run)
+{
+	const u32 bank = vector / 32;


the compiler should optimize this, but maybe >>5 would be good here?

Sure. This is a good improvement. I will update it.

romank-msft · 2025-05-29T19:01:39Z

drivers/hv/mshv_vtl_main.c

+static void do_assert_single_proxy_intr(const u32 vector, struct mshv_vtl_run *run)
+{
+	const u32 bank = vector / 32;
+	const u32 masked_irr = BIT(vector % 32) & ~READ_ONCE(run->proxy_irr_blocked[bank]);


& 0x1f instead of the modulo? Thinkling if the compiler doesn't optimize, that's the hot path

Sure. Good suggestion. I will add it.

arch/x86/kernel/cpu/mshyperv.c

ricardon · 2025-05-31T00:48:40Z

I updated the patches to incorporate feedback from @romank-msft and use a seqcount to serialize access to the redirected proxy interrupts data structures.

romank-msft · 2025-06-03T23:14:50Z

LGTM. Someone more familiar with the intent might want to take a look.

The computation of the proxy interrupt number needs to be performed each time is asserted. Optimize the computation using bit shifts and masks. While here, fix a minor issue on a comment. Suggested-by: Roman Kisel <[email protected]> Signed-off-by: Ricardo Neri <[email protected]> --- Changes since v2: * None Changes since v1: * Introduced this patch.

mshv_tdx_set_cpumask_from_apicid() does two things: it traverses the APICID hashtable and sets a CPU mask. Traversing the hastable can be carved out into a helper function. This makes the code more modular and other functions can benefit from such helper. Subsequent patches will make use of it. Signed-off-by: Ricardo Neri <[email protected]> --- Changes since v2: * Introduced this patch. Changes since v1: * N/A

The statically assigned interrupt vector of the VMBus synthetic interrupt is used to also assert proxy interrupts. These can be asserted in group or individually. Proxy interrupts can also be asserted individually if a redirection is in place to use a dedicated Linux IRQ. Add a helper function to assert individual proxy interrupts. No functional change intended. Signed-off-by: Ricardo Neri <[email protected]> --- Changes since v2: * None Changes since v1: * Rebased on the previous changeset that optimizes the calculation of the proxy interrupt bit.

The proxy interrupts that the VTL2 kernel relays to the VTL0 guest are handled via the single interrupt vector of the VMBus synthetic interrupt controller. On TDX guests in particular, Hyper-V does not use IOMMU posted interrupts for these interrupts. This results in a TDEXIT for every interrupt and consequent performance degradation. To recoup performance, newer versions of Hyper-V feature support the posting of VTL0 interrupts directly to the VTL2 guest. In such setup, the VTL2 guest still relays interrupts to the VTL0 guest, but uses a dedicated VTL2 interrupt vector for each VTL0 interrupt. The dedicated vector must be reserved only in one CPU that the user space specifies. Hyper-V shall post the interrupt only to the designated VTL2 vCPU. The VTL2 kernel must not migrate the dedicated vector to another VTL2 vCPU. User space in the VTL2 guest is responsible of requesting such redirection. Implement the new MAP_REDIRECTED_DEVICE_INTERRUPT IOCTL. User space provides the vector number of the VTL0 guest that wants the VTL2 kernel to redirect as well as the APIC ID of the VTL2 vCPU on which the interrupt redirection will take place. On success, return in the payload of the IOCTL the VTL2 CPU vector number reserved for this purpose. This is the vector number that Hyper-V must use when setting up the posted interrupt. Reuse the same IOCTL to also request the removal of the redirection when needed. When removing a redirection user space must provide the number of the VTL2 interrupt vector number that the kernel provided when the redirection was established. Signed-off-by: Ricardo Neri <[email protected]> --- Changes since v2: * Modified the payload of the IOCTL to also include the APIC ID of the VTL2 CPU on which the interrupt redirection will take place. * Removed weak stubs that are no longer needed. Changes since v1: * Assigned 0x39 to MAP_REDIRECTED_DEVICE_INTERRUPT as 0x38 was already taken. (Roman) * Fixed error handling for copy_to_user(). (Roman) * Moved comment about unmapping above the conditional statement. (Roman)

Add a thin irq_domain for redirected interrupts. The kernel IRQ infrastructure will use this domain to manage the resources and configuration of Linux IRQs to meet the requirements of redirected proxy interrupts. The redirected_intr_domain irq_domain defines the .alloc() operation to request the allocation of IRQs in the IRQ hierarchy all the way up to x86_vector_domain. Use dummy_irq_chip as the irq_chip for the domain since no other hardware configuration is needed. Importantly, this irq_chip does not define the .irq_set_affinity() operation. Redirected proxy interrupts do not allow changes in affinity. Signed-off-by: Ricardo Neri <[email protected]> --- Changes since v2: * Introduced this patch. Changes since v1: * N/A

Implement the setup portion of the MAP_REDIRECTED_DEVICE_INTERRUPT IOCTL. To redirect a proxy interrupt, allocate and request a Linux IRQ. Set the CPU affinity of the IRQ to that user space specified. Also, specify to request_irq() that the interrupt should be excluded from interrupt balancing as such action would change the CPU affinity of the interrupt. The contract with user space restricts the kernel from changing it. Upon getting an interrupt, the handler will assert the proxy interrupt vector that user space specified. Remember which hardware vector (i.e., the CPU vector number) that was assigned to the IRQ. This information is needed to tear down the redirection of the proxy interrupt (it will be implemented in a subsequent changset). Keep a list of the redirected interrupts. Use it to avoid redirecting the same proxy interrupt more than once. Signed-off-by: Ricardo Neri <[email protected]> --- Changes since v2: * Introduced this patch. Changes since v1: * N/A

User space requests the tear down of a redirected interrupt using the APIC ID and the CPU interrupt vector number associated with it. Traverse the list of redirected interrupt and release the resources of the IRQ being teared down. Signed-off-by: Ricardo Neri <[email protected]> --- Changes since v2: * Introduced this patch. Changes since v1: * N/A

ricardon · 2025-10-31T22:46:49Z

I have updated this pull request to not touch the arch/x86 directory and instead use the existing Linux IRQ infrastructure.

ricardon · 2025-11-06T17:55:41Z

@romank-msft @chris-oo would you mind taking a look at these changes when you get a chance? These are the kernel changes that microsoft/openvmm#384 from @balajimc55

chris-oo

mostly looks good, just had a question on when we should enable this functionality

drivers/hv/mshv_vtl_main.c

namancse · 2025-11-13T16:11:23Z

Changes look good to me. They are well separated into different commits, and the commit messages make it easy to understand the purpose and implementation.

namancse

Looks good to me.

ricardon · 2025-11-14T22:04:56Z

Looks good to me.

Thank you @namancse !

hargar19 · 2025-11-14T23:44:14Z

@ricardon Please merge this PR when you're ready

ricardon · 2025-11-15T01:29:27Z

@hargar19 I don't think I have permissions to merge PRs.

ricardon mentioned this pull request May 27, 2025

Add support in L1 kernel handle interrupts posted from the IOMMU microsoft/openvmm#406

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chris-oo self-assigned this May 28, 2025

romank-msft reviewed May 29, 2025

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arch/x86/kernel/cpu/mshyperv.c Outdated Show resolved Hide resolved

ricardon force-pushed the rneri/redirec-posted-intr branch 2 times, most recently from 76e36f5 to 9fe424d Compare May 31, 2025 00:46

romank-msft approved these changes Jun 3, 2025

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ricardon added 7 commits October 31, 2025 08:48

ricardon force-pushed the rneri/redirec-posted-intr branch from 9fe424d to 9f0c09e Compare October 31, 2025 19:53

chris-oo reviewed Nov 7, 2025

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drivers/hv/mshv_vtl_main.c Show resolved Hide resolved

namancse self-requested a review November 13, 2025 16:09

namancse reviewed Nov 13, 2025

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ricardon closed this Nov 15, 2025

ricardon reopened this Nov 15, 2025

hargar19 merged commit 4e114da into microsoft:product/hcl-main/6.12 Nov 17, 2025
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Add support for redirected VTL0 interrupts #80

Add support for redirected VTL0 interrupts #80

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