Re: [PATCH 08/15] arm/arm64: KVM: Always have merged page tables

Christoffer Dall
2016-06-30T13:28:17Z

On Thu, Jun 30, 2016 at 01:27:05PM +0100, Marc Zyngier wrote:
> On 28/06/16 22:43, Christoffer Dall wrote:
> > On Tue, Jun 07, 2016 at 11:58:28AM +0100, Marc Zyngier wrote:
> >> We're in a position where we can now always have "merged" page
> >> tables, where both the runtime mapping and the idmap coexist.
> >>
> >> This results in some code being removed, but there is more to come.
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >> ---
> >> arch/arm/kvm/mmu.c | 74 +++++++++++++++++++++++---------------------------
> >> arch/arm64/kvm/reset.c | 31 +++++----------------
> >> 2 files changed, 41 insertions(+), 64 deletions(-)
> >>
> >> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> >> index d6ecbf1..9a17e14 100644
> >> --- a/arch/arm/kvm/mmu.c
> >> +++ b/arch/arm/kvm/mmu.c
> >> @@ -492,13 +492,12 @@ void free_boot_hyp_pgd(void)
> >>
> >> if (boot_hyp_pgd) {
> >> unmap_hyp_range(boot_hyp_pgd, hyp_idmap_start, PAGE_SIZE);
> >> - unmap_hyp_range(boot_hyp_pgd, TRAMPOLINE_VA, PAGE_SIZE);
> >> free_pages((unsigned long)boot_hyp_pgd, hyp_pgd_order);
> >> boot_hyp_pgd = NULL;
> >> }
> >>
> >> if (hyp_pgd)
> >> - unmap_hyp_range(hyp_pgd, TRAMPOLINE_VA, PAGE_SIZE);
> >> + unmap_hyp_range(hyp_pgd, hyp_idmap_start, PAGE_SIZE);
> >>
> >> mutex_unlock(&kvm_hyp_pgd_mutex);
> >> }
> >> @@ -1690,7 +1689,7 @@ phys_addr_t kvm_mmu_get_boot_httbr(void)
> >> if (__kvm_cpu_uses_extended_idmap())
> >> return virt_to_phys(merged_hyp_pgd);
> >> else
> >> - return virt_to_phys(boot_hyp_pgd);
> >> + return virt_to_phys(hyp_pgd);
> >> }
> >>
> >> phys_addr_t kvm_get_idmap_vector(void)
> >> @@ -1703,6 +1702,22 @@ phys_addr_t kvm_get_idmap_start(void)
> >> return hyp_idmap_start;
> >> }
> >>
> >> +static int kvm_map_idmap_text(pgd_t *pgd)
> >> +{
> >> + int err;
> >> +
> >> + /* Create the idmap in the boot page tables */
> >> + err = __create_hyp_mappings(pgd,
> >> + hyp_idmap_start, hyp_idmap_end,
> >> + __phys_to_pfn(hyp_idmap_start),
> >> + PAGE_HYP);
> >> + if (err)
> >> + kvm_err("Failed to idmap %lx-%lx\n",
> >> + hyp_idmap_start, hyp_idmap_end);
> >> +
> >> + return err;
> >> +}
> >> +
> >> int kvm_mmu_init(void)
> >> {
> >> int err;
> >> @@ -1718,27 +1733,25 @@ int kvm_mmu_init(void)
> >> BUG_ON((hyp_idmap_start ^ (hyp_idmap_end - 1)) & PAGE_MASK);
> >>
> >> hyp_pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, hyp_pgd_order);
> >> - boot_hyp_pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, hyp_pgd_order);
> >> -
> >> - if (!hyp_pgd || !boot_hyp_pgd) {
> >> + if (!hyp_pgd) {
> >> kvm_err("Hyp mode PGD not allocated\n");
> >> err = -ENOMEM;
> >> goto out;
> >> }
> >>
> >> - /* Create the idmap in the boot page tables */
> >> - err = __create_hyp_mappings(boot_hyp_pgd,
> >> - hyp_idmap_start, hyp_idmap_end,
> >> - __phys_to_pfn(hyp_idmap_start),
> >> - PAGE_HYP);
> >> + if (__kvm_cpu_uses_extended_idmap()) {
> >> + boot_hyp_pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
> >> + hyp_pgd_order);
> >> + if (!boot_hyp_pgd) {
> >> + kvm_err("Hyp boot PGD not allocated\n");
> >> + err = -ENOMEM;
> >> + goto out;
> >> + }
> >>
> >> - if (err) {
> >> - kvm_err("Failed to idmap %lx-%lx\n",
> >> - hyp_idmap_start, hyp_idmap_end);
> >> - goto out;
> >> - }
> >> + err = kvm_map_idmap_text(boot_hyp_pgd);
> >> + if (err)
> >> + goto out;
> >>
> >> - if (__kvm_cpu_uses_extended_idmap()) {
> >> merged_hyp_pgd = (pgd_t *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> >> if (!merged_hyp_pgd) {
> >> kvm_err("Failed to allocate extra HYP pgd\n");
> >> @@ -1746,29 +1759,10 @@ int kvm_mmu_init(void)
> >> }
> >> __kvm_extend_hypmap(boot_hyp_pgd, hyp_pgd, merged_hyp_pgd,
> >> hyp_idmap_start);
> >> - return 0;
> >> - }
> >> -
> >> - /* Map the very same page at the trampoline VA */
> >> - err = __create_hyp_mappings(boot_hyp_pgd,
> >> - TRAMPOLINE_VA, TRAMPOLINE_VA + PAGE_SIZE,
> >> - __phys_to_pfn(hyp_idmap_start),
> >> - PAGE_HYP);
> >> - if (err) {
> >> - kvm_err("Failed to map trampoline @%lx into boot HYP pgd\n",
> >> - TRAMPOLINE_VA);
> >> - goto out;
> >> - }
> >> -
> >> - /* Map the same page again into the runtime page tables */
> >> - err = __create_hyp_mappings(hyp_pgd,
> >> - TRAMPOLINE_VA, TRAMPOLINE_VA + PAGE_SIZE,
> >> - __phys_to_pfn(hyp_idmap_start),
> >> - PAGE_HYP);
> >> - if (err) {
> >> - kvm_err("Failed to map trampoline @%lx into runtime HYP pgd\n",
> >> - TRAMPOLINE_VA);
> >> - goto out;
> >> + } else {
> >> + err = kvm_map_idmap_text(hyp_pgd);
> >> + if (err)
> >> + goto out;
> >
> > Something I'm not clear on:
> >
> > how can we always have merged pgtables on 32-bit ARM at this point?
> >
> > why is there not a potential conflict at this point in the series
> > between the runtime hyp mappings and the idmaps?
>
> The problem is slightly different. On 32bit, the HYP mapping completely
> covers the whole address space, just like the kernel. But if your idmap
> and the kernel VA overlap, you are in a very weird position. Actually,
> you'll even have trouble booting into the kernel.
>
> So my take on this is that it has already been solved by making sure the
> kernel is loaded at an address that won't alias with the kernel VA. If
> it hasn't, then they are probably not running mainline Linux.
>
Ok, thanks for the explanation!

-Christoffer