Re: [PATCH] mshv: Simplify GPA map/unmap hypercall helpers

[Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>]

On Wed, Apr 29, 2026 at 11:02:37AM +0000, Anirudh Rayabharam wrote:
> On Tue, Apr 28, 2026 at 11:21:12PM +0000, Stanislav Kinsburskii wrote:
> > Clean up hv_do_map_gpa_hcall() and hv_call_unmap_gpa_pages() after the
> > preceding bug-fix patches:
> > 
> > Move "done += completed" before the status checks so that pages mapped
> > by a partially-successful batch are included in the error cleanup unmap.
> > Previously these mappings were leaked on failure.
> > 
> > While here, improve type safety and readability:
> >  - Change "int done" to "u64 done" to match the u64 page_count it is
> >    compared against, avoiding signed/unsigned comparison hazards.
> >  - Use u64 for loop iteration and batch size variables consistently.
> >  - Add proper braces to the for-loop body in hv_do_map_gpa_hcall().
> >  - Remove unnecessary "ret" variable from hv_call_unmap_gpa_pages().
> >  - Simplify the error-path unmap to use "done << large_shift" directly
> >    instead of mutating done in place.
> > 
> > Fixes: 621191d709b14 ("Drivers: hv: Introduce mshv_root module to expose /dev/mshv to VMMs")
> > Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
> > ---
> >  drivers/hv/mshv_root_hv_call.c |   55 +++++++++++++++-------------------------
> >  1 file changed, 20 insertions(+), 35 deletions(-)
> > 
> > diff --git a/drivers/hv/mshv_root_hv_call.c b/drivers/hv/mshv_root_hv_call.c
> > index e5992c324904a..f5f205a397834 100644
> > --- a/drivers/hv/mshv_root_hv_call.c
> > +++ b/drivers/hv/mshv_root_hv_call.c
> > @@ -195,8 +195,8 @@ static int hv_do_map_gpa_hcall(u64 partition_id, u64 gfn, u64 page_struct_count,
> >  	struct hv_input_map_gpa_pages *input_page;
> >  	u64 status, *pfnlist;
> >  	unsigned long irq_flags, large_shift = 0;
> > -	int ret = 0, done = 0;
> > -	u64 page_count = page_struct_count;
> > +	u64 done = 0, page_count = page_struct_count;
> > +	int ret = 0;
> >  
> >  	if (page_count == 0 || (pages && mmio_spa))
> >  		return -EINVAL;
> > @@ -213,8 +213,8 @@ static int hv_do_map_gpa_hcall(u64 partition_id, u64 gfn, u64 page_struct_count,
> >  	}
> >  
> >  	while (done < page_count) {
> > -		ulong i, completed, remain = page_count - done;
> > -		int rep_count = min(remain, HV_MAP_GPA_BATCH_SIZE);
> > +		u64 i, completed, remain = page_count - done;
> > +		u64 rep_count = min(remain, (u64)HV_MAP_GPA_BATCH_SIZE);
> >  
> >  		local_irq_save(irq_flags);
> >  		input_page = *this_cpu_ptr(hyperv_pcpu_input_arg);
> > @@ -224,23 +224,13 @@ static int hv_do_map_gpa_hcall(u64 partition_id, u64 gfn, u64 page_struct_count,
> >  		input_page->map_flags = flags;
> >  		pfnlist = input_page->source_gpa_page_list;
> >  
> > -		for (i = 0; i < rep_count; i++)
> > -			if (flags & HV_MAP_GPA_NO_ACCESS) {
> > +		for (i = 0; i < rep_count; i++) {
> > +			if (flags & HV_MAP_GPA_NO_ACCESS)
> >  				pfnlist[i] = 0;
> > -			} else if (pages) {
> > -				u64 index = (done + i) << large_shift;
> > -
> > -				if (index >= page_struct_count) {
> > -					ret = -EINVAL;
> > -					break;
> > -				}
> > -				pfnlist[i] = page_to_pfn(pages[index]);
> > -			} else {
> > +			else if (pages)
> > +				pfnlist[i] = page_to_pfn(pages[(done + i) << large_shift]);
> > +			else
> >  				pfnlist[i] = mmio_spa + done + i;
> > -			}
> > -		if (ret) {
> > -			local_irq_restore(irq_flags);
> > -			break;
> >  		}
> >  
> >  		status = hv_do_rep_hypercall(HVCALL_MAP_GPA_PAGES, rep_count, 0,
> > @@ -248,29 +238,26 @@ static int hv_do_map_gpa_hcall(u64 partition_id, u64 gfn, u64 page_struct_count,
> >  		local_irq_restore(irq_flags);
> >  
> >  		completed = hv_repcomp(status);
> > +		done += completed;
> >  
> >  		if (hv_result_needs_memory(status)) {
> >  			ret = hv_call_deposit_pages(NUMA_NO_NODE, partition_id,
> >  						    HV_MAP_GPA_DEPOSIT_PAGES);
> >  			if (ret)
> >  				break;
> > -
> >  		} else if (!hv_result_success(status)) {
> >  			ret = hv_result_to_errno(status);
> >  			break;
> >  		}
> > -
> > -		done += completed;
> >  	}
> >  
> >  	if (ret && done) {
> >  		u32 unmap_flags = 0;
> >  
> > -		if (flags & HV_MAP_GPA_LARGE_PAGE) {
> > +		if (flags & HV_MAP_GPA_LARGE_PAGE)
> >  			unmap_flags |= HV_UNMAP_GPA_LARGE_PAGE;
> > -			done <<= large_shift;
> > -		}
> > -		hv_call_unmap_gpa_pages(partition_id, gfn, done, unmap_flags);
> > +		hv_call_unmap_gpa_pages(partition_id, gfn,
> > +					done << large_shift, unmap_flags);
> 
> How does this work? Earlier we were doing "done << large_shift" only if
> HV_MAP_GPA_LARGE_PAGE is set but now we always do it.
> 

It works becuase large_shift in initialized to 0 when
HV_MAP_GPA_LARGE_PAGE is not set.

Thanks,
Stanislav

> Thanks,
> Anirudh.