On 2/26/2025 5:50 PM, Michael Kelley wrote: > From: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx> Sent: Wednesday, February 26, 2025 4:15 PM >> >> On 2/26/2025 12:06 PM, mhkelley58@xxxxxxxxx wrote: >>> From: Michael Kelley <mhklinux@xxxxxxxxxxx> >>> >>> Current code allocates the "hyperv_pcpu_input_arg", and in >>> some configurations, the "hyperv_pcpu_output_arg". Each is a 4 KiB >>> page of memory allocated per-vCPU. A hypercall call site disables >>> interrupts, then uses this memory to set up the input parameters for >>> the hypercall, read the output results after hypercall execution, and >>> re-enable interrupts. The open coding of these steps leads to >>> inconsistencies, and in some cases, violation of the generic >>> requirements for the hypercall input and output as described in the >>> Hyper-V Top Level Functional Spec (TLFS)[1]. >>> >>> To reduce these kinds of problems, introduce a family of inline >>> functions to replace the open coding. The functions provide a new way >>> to manage the use of this per-vCPU memory that is usually the input and >>> output arguments to Hyper-V hypercalls. The functions encapsulate >>> key aspects of the usage and ensure that the TLFS requirements are >>> met (max size of 1 page each for input and output, no overlap of >>> input and output, aligned to 8 bytes, etc.). Conceptually, there >>> is no longer a difference between the "per-vCPU input page" and >>> "per-vCPU output page". Only a single per-vCPU page is allocated, and >>> it provides both hypercall input and output memory. All current >>> hypercalls can fit their input and output within that single page, >>> though the new code allows easy changing to two pages should a future >>> hypercall require a full page for each of the input and output. >>> >>> The new functions always zero the fixed-size portion of the hypercall >>> input area so that uninitialized memory is not inadvertently passed >>> to the hypercall. Current open-coded hypercall call sites are >>> inconsistent on this point, and use of the new functions addresses >>> that inconsistency. The output area is not zero'ed by the new code >>> as it is Hyper-V's responsibility to provide legal output. >>> >>> When the input or output (or both) contain an array, the new functions >>> calculate and return how many array entries fit within the per-cpu >>> memory page, which is effectively the "batch size" for the hypercall >>> processing multiple entries. This batch size can then be used in the >>> hypercall control word to specify the repetition count. This >>> calculation of the batch size replaces current open coding of the >>> batch size, which is prone to errors. Note that the array portion of >>> the input area is *not* zero'ed. The arrays are almost always 64-bit >>> GPAs or something similar, and zero'ing that much memory seems >>> wasteful at runtime when it will all be overwritten. The hypercall >>> call site is responsible for ensuring that no part of the array is >>> left uninitialized (just as with current code). >>> >>> The new functions are realized as a single inline function that >>> handles the most complex case, which is a hypercall with input >>> and output, both of which contain arrays. Simpler cases are mapped to >>> this most complex case with #define wrappers that provide zero or NULL >>> for some arguments. Several of the arguments to this new function are >>> expected to be compile-time constants generated by "sizeof()" >>> expressions. As such, most of the code in the new function can be >>> evaluated by the compiler, with the result that the code paths are >>> no longer than with the current open coding. The one exception is >>> new code generated to zero the fixed-size portion of the input area >>> in cases where it is not currently done. >>> >>> [1] https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/tlfs >>> >>> Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx> >>> --- >>> include/asm-generic/mshyperv.h | 102 +++++++++++++++++++++++++++++++++ >>> 1 file changed, 102 insertions(+) >>> >>> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h >>> index b13b0cda4ac8..0c8a9133cf1a 100644 >>> --- a/include/asm-generic/mshyperv.h >>> +++ b/include/asm-generic/mshyperv.h >>> @@ -135,6 +135,108 @@ static inline u64 hv_do_rep_hypercall(u16 code, u16 rep_count, u16 varhead_size, >>> return status; >>> } >>> >>> +/* >>> + * Hypercall input and output argument setup >>> + */ >>> + >>> +/* Temporary mapping to be removed at the end of the patch series */ >>> +#define hyperv_pcpu_arg hyperv_pcpu_input_arg >>> + >>> +/* >>> + * Allocate one page that is shared between input and output args, which is >>> + * sufficient for all current hypercalls. If a future hypercall requires >>> + * more space, change this value to "2" and everything will work. >>> + */ >>> +#define HV_HVCALL_ARG_PAGES 1 >>> + >>> +/* >>> + * Allocate space for hypercall input and output arguments from the >>> + * pre-allocated per-cpu hyperv_pcpu_args page(s). A NULL value for the input >>> + * or output indicates to allocate no space for that argument. For input and >>> + * for output, specify the size of the fixed portion, and the size of an >>> + * element in a variable size array. A zero value for entry_size indicates >>> + * there is no array. The fixed size space for the input is zero'ed. >>> + * >> It might be worth explicitly mentioning that interrupts should be disabled when >> calling this function. > > Agreed. > >> >>> + * When variable size arrays are present, the function returns the number of >>> + * elements (i.e, the batch size) that fit in the available space. >>> + * >>> + * The four "size" arguments are expected to be constants, in which case the >>> + * compiler does most of the calculations. Then the generated inline code is no >>> + * larger than if open coding the access to the hyperv_pcpu_arg and doing >>> + * memset() on the input. >>> + */ >>> +static inline int hv_hvcall_inout_array( >>> + void *input, u32 in_size, u32 in_entry_size, >>> + void *output, u32 out_size, u32 out_entry_size) >> Is there a reason input and output are void * instead of void ** here? > > Yes -- it must be void *, and not void **. Consider a function like get_vtl() > in Patch 3 of this series where local variable "input" is declared as: > > struct hv_input_get_vp_registers *input; > > Then the first argument to hv_hvcall_inout() will be of type > struct hv_input_get_vp_registers **. The compiler does not consider > this to match void ** and would generate an error. But > struct hv_input_get_vp_registers ** _does_ match void * and compiles > with no error. It makes sense when you think about it, though it > isn't obvious. I tried to use void ** initially and had to figure out > why the code wouldn't compile. :-) > Ah, that does make sense. Okay then, fair enough! >> >>> +{ >>> + u32 in_batch_count = 0, out_batch_count = 0, batch_count; >>> + u32 in_total_size, out_total_size, offset; >>> + u32 batch_space = HV_HYP_PAGE_SIZE * HV_HVCALL_ARG_PAGES; >>> + void *space; >>> + >>> + /* >>> + * If input and output have arrays, allocate half the space to input >>> + * and half to output. If only input has an array, the array can use >>> + * all the space except for the fixed size output (but not to exceed >>> + * one page), and vice versa. >>> + */ >>> + if (in_entry_size && out_entry_size) >>> + batch_space = batch_space / 2; >>> + else if (in_entry_size) >>> + batch_space = min(HV_HYP_PAGE_SIZE, batch_space - out_size); >>> + else if (out_entry_size) >>> + batch_space = min(HV_HYP_PAGE_SIZE, batch_space - in_size); >>> + >>> + if (in_entry_size) >>> + in_batch_count = (batch_space - in_size) / in_entry_size; >>> + if (out_entry_size) >>> + out_batch_count = (batch_space - out_size) / out_entry_size; >>> + >>> + /* >>> + * If input and output have arrays, use the smaller of the two batch >>> + * counts, in case they are different. If only one has an array, use >>> + * that batch count. batch_count will be zero if neither has an array. >>> + */ >>> + if (in_batch_count && out_batch_count) >>> + batch_count = min(in_batch_count, out_batch_count); >>> + else >>> + batch_count = in_batch_count | out_batch_count; >>> + >>> + in_total_size = ALIGN(in_size + (in_entry_size * batch_count), 8); >>> + out_total_size = ALIGN(out_size + (out_entry_size * batch_count), 8); >>> + >>> + space = *this_cpu_ptr(hyperv_pcpu_arg); >>> + if (input) { >>> + *(void **)input = space; >>> + if (space) >>> + /* Zero the fixed size portion, not any array portion */ >>> + memset(space, 0, ALIGN(in_size, 8)); >>> + } >>> + >>> + if (output) { >>> + if (in_total_size + out_total_size <= HV_HYP_PAGE_SIZE) { >>> + offset = in_total_size; >>> + } else { >>> + offset = HV_HYP_PAGE_SIZE; >>> + /* Need more than 1 page, but only 1 was allocated */ >>> + BUILD_BUG_ON(HV_HVCALL_ARG_PAGES == 1); >> Interesting... so the compiler is not compiling this BUILD_BUG_ON in your patchset >> because in_total_size + out_total_size <= HV_HYP_PAGE_SIZE is always known at >> compile-time? > > Correct. And if for some future hypercall in_total_size + out_total_size is > *not* <= HV_HYP_PAGE_SIZE, the BUILD_BUG_ON() will alert the developer > to the problem. Depending on the argument requirements of this future > hypercall, the solution might require changing HV_HVCALL_ARG_PAGES to 2. > >> So will this also fail if any of the args in_size, in_entry_size, out_size, >> out_entry_size are runtime-known? > > Correct. I should change my wording about "The four 'size' arguments are > expected to be constants" to ". . . must be constants". OTOH, if there's a need > to support non-constants for any of these arguments, some additional code > could handle that case. But the overall hv_hvcall_inout_array() function will > end up generating a lot of code to execute at runtime. I looked at the hypercall > call sites in the OHCL kernel tree, and didn't see any need for non-constants, > but I haven't looked yet at the v4 patch series you just posted. Let me know > if you have a case requiring non-constants. > I don't think we ever use non-constants. In fact I can't think of a case where these values aren't the result of a sizeof() (or 0). Overall I think this approach is looking quite nice and I'd be happy to adopt it in the mshv driver code when this is merged. With the comment change mentioned above: Reviewed-by: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx> > Michael
