Re: [PATCH 4/4] mshv: Pre-deposit pages for SLAT creation

[Mukesh R <mrathor@linux.microsoft.com>]

On 3/3/26 16:23, Stanislav Kinsburskii wrote:
> Deposit enough pages up front to avoid guest address space region creation
> failures due to low memory. This also speeds up guest creation.
> 
> Calculate the required number of pages based on the guest's physical
> address space size, rounded up to 1 GB chunks. Even the smallest guests are
> assumed to need at least 1 GB worth of deposits. This is because every
> guest requires tens of megabytes of deposited pages for hypervisor
> overhead, making smaller deposits impractical.
> 
> Estimating in 1 GB chunks prevents over-depositing for larger guests while
> accepting some over-deposit for smaller ones. This trade-off keeps the
> estimate close to actual needs for larger guests.
> 
> Also withdraw the deposited pages if address space region creation fails.
> 
> Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
> ---
>   drivers/hv/mshv_root_main.c |   25 +++++++++++++++++++++++--
>   1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c
> index 48c842b6938d..cb5b4505f8eb 100644
> --- a/drivers/hv/mshv_root_main.c
> +++ b/drivers/hv/mshv_root_main.c
> @@ -39,6 +39,7 @@
>   #define MSHV_PARTITION_DEPOSIT_PAGES		(SZ_512K >> PAGE_SHIFT)
>   #define MSHV_PARTITION_DEPOSIT_PAGES_NESTED	(20 * SZ_1M >> PAGE_SHIFT)
>   #define MSHV_VP_DEPOSIT_PAGES			(1 * SZ_1M >> PAGE_SHIFT)
> +#define MSHV_1G_DEPOSIT_PAGES			(6 * SZ_1M >> PAGE_SHIFT)
>   
>   MODULE_AUTHOR("Microsoft");
>   MODULE_LICENSE("GPL");
> @@ -1324,6 +1325,18 @@ static int mshv_prepare_pinned_region(struct mshv_mem_region *region)
>   	return ret;
>   }
>   
> +static u64
> +mshv_region_deposit_slat_pages(struct mshv_mem_region *region)
> +{
> +	u64 region_in_gbs, slat_pages;
> +
> +	/* SLAT needs 6 MB per 1 GB of address space. */
> +	region_in_gbs = DIV_ROUND_UP(region->nr_pages << HV_HYP_PAGE_SHIFT, SZ_1G);
> +	slat_pages = region_in_gbs * MSHV_1G_DEPOSIT_PAGES;
> +
> +	return slat_pages;
> +}
> +
>   /*
>    * This maps two things: guest RAM and for pci passthru mmio space.
>    *
> @@ -1364,6 +1377,11 @@ mshv_map_user_memory(struct mshv_partition *partition,
>   	if (ret)
>   		return ret;
>   
> +	ret = hv_call_deposit_pages(NUMA_NO_NODE, partition->pt_id,
> +				    mshv_region_deposit_slat_pages(region));
> +	if (ret)
> +		goto free_region;
> +


Also, for MSHV_REGION_TYPE_MEM_PINNED, deposit is not needed.


>   	switch (region->mreg_type) {
>   	case MSHV_REGION_TYPE_MEM_PINNED:
>   		ret = mshv_prepare_pinned_region(region);
> @@ -1392,7 +1410,7 @@ mshv_map_user_memory(struct mshv_partition *partition,
>   				   region->hv_map_flags, ret);
>   
>   	if (ret)
> -		goto errout;
> +		goto withdraw_memory;
>   
>   	spin_lock(&partition->pt_mem_regions_lock);
>   	hlist_add_head(&region->hnode, &partition->pt_mem_regions);
> @@ -1400,7 +1418,10 @@ mshv_map_user_memory(struct mshv_partition *partition,
>   
>   	return 0;
>   
> -errout:
> +withdraw_memory:
> +	hv_call_withdraw_memory(mshv_region_deposit_slat_pages(region),
> +				NUMA_NO_NODE, partition->pt_id);
> +free_region:
>   	vfree(region);
>   	return ret;
>   }
> 
>