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x86/hyperv: Add comments about hv_vpset and var size hypercall input args

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Current code varies in how the size of the variable size input header
for hypercalls is calculated when the input contains struct hv_vpset.
Surprisingly, this variation is correct, as different hypercalls make
different choices for what portion of struct hv_vpset is treated as part
of the variable size input header. The Hyper-V TLFS is silent on these
details, but the behavior has been confirmed with Hyper-V developers.

To avoid future confusion about these differences, add comments to
struct hv_vpset, and to hypercall call sites with input that contains
a struct hv_vpset. The comments describe the overall situation and
the calculation that should be used at each particular call site.

No functional change as only comments are updated.

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Link: https://lore.kernel.org/r/20250318214919.958953-1-mhklinux@outlook.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250318214919.958953-1-mhklinux@outlook.com>
This commit is contained in:
Michael Kelley 2025-03-18 14:49:19 -07:00 • committed by Wei Liu
parent 621191d709
commit 999ad14259
3 changed files with 17 additions and 1 deletions
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5
arch/x86/hyperv/hv_apic.c
View file

@ -145,6 +145,11 @@ static bool __send_ipi_mask_ex(const struct cpumask *mask, int vector,
ipi_arg->vp_set.format = HV_GENERIC_SET_ALL;
}
/*
* For this hypercall, Hyper-V treats the valid_bank_mask field
* of ipi_arg->vp_set as part of the fixed size input header.
* So the variable input header size is equal to nr_bank.
*/
status = hv_do_rep_hypercall(HVCALL_SEND_IPI_EX, 0, nr_bank,
ipi_arg, NULL);

4
arch/x86/hyperv/mmu.c
View file

@ -205,6 +205,10 @@ static u64 hyperv_flush_tlb_others_ex(const struct cpumask *cpus,
/*
* We can flush not more than max_gvas with one hypercall. Flush the
* whole address space if we were asked to do more.
*
* For these hypercalls, Hyper-V treats the valid_bank_mask field
* of flush->hv_vp_set as part of the fixed size input header.
* So the variable input header size is equal to nr_bank.
*/
max_gvas =
(PAGE_SIZE - sizeof(*flush) - nr_bank *

9
include/hyperv/hvgdk_mini.h
View file

@ -205,7 +205,14 @@ union hv_reference_tsc_msr {
/* The number of vCPUs in one sparse bank */
#define HV_VCPUS_PER_SPARSE_BANK (64)
/* Some of Hyper-V structs do not use hv_vpset where linux uses them */
/*
* Some of Hyper-V structs do not use hv_vpset where linux uses them.
*
* struct hv_vpset is usually used as part of hypercall input. The portion
* that counts as "fixed size input header" vs. "variable size input header"
* varies per hypercall. See comments at relevant hypercall call sites as to
* how the "valid_bank_mask" field should be accounted.
*/
struct hv_vpset { /* HV_VP_SET */
u64 format;
u64 valid_bank_mask;