Re: [PATCH v4 1/8] KVM: s390: pv: Use VM_SPARSE area for guest variable storage area

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 20.07.26 um 10:58 schrieb Heiko Carstens:
> The guest variable storage area is allocated with vmalloc and then
> donated to the ultravisor. Any kernel access to that area will result
> in a secure storage access exception (aka fault).
> 
> This is a problem if such a memory area is read via /proc/kcore. This
> causes an exception via vread_iter() and results in an unexpected short
> read. Avoid this by allocating a custom VM_SPARSE area. If such an area
> is read, vread_iter() returns zeroes for the entire area.
> 
> Note that the function which frees the area does not update ptes. This
> is intentional to allow for deferred / lazy pte updates and TLB flushing
> like the generic vfree() code is doing that. See vunmap_pte_range().
> 
> This assumes that s390 will gain full support for lazy_mmu_mode_enable()
> and lazy_mmu_mode_disable() in the future, since as of now the used
> ptep_get_and_clear() in vunmap_pte_range() does indeed invalidate and
> flush every single pte entry, but only for s390.
> 
> Tested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>


[..]> +static int uv_free_range_cb(pte_t *ptep, unsigned long addr, void *data)
> +{
> +	pte_t pte = ptep_get(ptep);
> +
> +	if (!pte_present(pte))
> +		return 0;
I guess we do not need it today, but it certainly does not hurt?