Re: [Xen-devel] [PATCH v2 39/55] x86: switch root_pgt to mfn_t and use new APIs

[Hongyan Xia <hongyax@xxxxxxxxxx>]

On 30/09/2019 11:33, Hongyan Xia wrote:
> From: Wei Liu <wei.liu2@xxxxxxxxxx>
>
> This then requires moving declaration of root page table mfn into mm.h
> and modify setup_cpu_root_pgt to have a single exit path.
>
> We also need to force map_domain_page to use direct map when switching
> per-domain mappings. This is contrary to our end goal of removing
> direct map, but this will be removed once we make map_domain_page
> context-switch safe in another (large) patch series.
>
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>   xen/arch/x86/domain.c           | 15 ++++++++++---
>   xen/arch/x86/domain_page.c      |  2 +-
>   xen/arch/x86/mm.c               |  2 +-
>   xen/arch/x86/pv/domain.c        |  2 +-
>   xen/arch/x86/smpboot.c          | 40 ++++++++++++++++++++++-----------
>   xen/include/asm-x86/mm.h        |  2 ++
>   xen/include/asm-x86/processor.h |  2 +-
>   7 files changed, 45 insertions(+), 20 deletions(-)
>
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index dbdf6b1bc2..e9bf47efce 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -69,6 +69,7 @@
>   #include <asm/pv/domain.h>
>   #include <asm/pv/mm.h>
>   #include <asm/spec_ctrl.h>
> +#include <asm/setup.h>
>   
>   DEFINE_PER_CPU(struct vcpu *, curr_vcpu);
>   
> @@ -1580,12 +1581,20 @@ void paravirt_ctxt_switch_from(struct vcpu *v)
>   
>   void paravirt_ctxt_switch_to(struct vcpu *v)
>   {
> -    root_pgentry_t *root_pgt = this_cpu(root_pgt);
> +    mfn_t rpt_mfn = this_cpu(root_pgt_mfn);
>   
> -    if ( root_pgt )
> -        root_pgt[root_table_offset(PERDOMAIN_VIRT_START)] =
> +    if ( !mfn_eq(rpt_mfn, INVALID_MFN) )
> +    {
> +        root_pgentry_t *rpt;
> +
> +        mapcache_override_current(INVALID_VCPU);
> +        rpt = map_xen_pagetable_new(rpt_mfn);
> +        rpt[root_table_offset(PERDOMAIN_VIRT_START)] =
>               l4e_from_page(v->domain->arch.perdomain_l3_pg,
>                             __PAGE_HYPERVISOR_RW);
> +        UNMAP_XEN_PAGETABLE_NEW(rpt);
> +        mapcache_override_current(NULL);
> +    }
>   
>       if ( unlikely(v->arch.dr7 & DR7_ACTIVE_MASK) )
>           activate_debugregs(v);
I am having second thoughts on whether I should include this patch for now. 
Obviously the per-domain mapcache in its current form cannot be used here 
during the context switch. However, I also don't want to use PMAP because it is 
just a bootstrapping mechanism and may result in heavy lock contention here.
I am inclined to drop it for now and include this after we have a 
context-switch safe mapping mechanism, as the commit message suggests.
Hongyan

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