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Re: [Qemu-devel] [PATCH 2/3] i386: Atomically update PTEs with mttcg
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From: |
Benjamin Herrenschmidt |
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Subject: |
Re: [Qemu-devel] [PATCH 2/3] i386: Atomically update PTEs with mttcg |
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Date: |
Fri, 14 Dec 2018 11:05:08 +1100 |
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User-agent: |
Evolution 3.30.2 (3.30.2-2.fc29) |
Note to RiscV folks: You may want to adapt your code to do the same as
this, esp. afaik, what you do today is endian-broken if host and guest
endian are different.
Cheers,
Ben.
On Fri, 2018-12-14 at 10:58 +1100, Benjamin Herrenschmidt wrote:
> Afaik, this isn't well documented (at least it wasn't when I last looked)
> but OSes such as Linux rely on this behaviour:
>
> The HW updates to the page tables need to be done atomically with the
> checking of the present bit (and other permissions).
>
> This is what allows Linux to do simple xchg of PTEs with 0 and assume
> the value read has "final" stable dirty and accessed bits (the TLB
> invalidation is deferred).
>
> Signed-off-by: Benjamin Herrenschmidt <address@hidden>
> ---
> target/i386/excp_helper.c | 104 +++++++++++++++++++++++++++++---------
> 1 file changed, 80 insertions(+), 24 deletions(-)
>
> diff --git a/target/i386/excp_helper.c b/target/i386/excp_helper.c
> index 49231f6b69..93fc24c011 100644
> --- a/target/i386/excp_helper.c
> +++ b/target/i386/excp_helper.c
> @@ -157,11 +157,45 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
>
> #else
>
> +static inline uint64_t update_entry(CPUState *cs, target_ulong addr,
> + uint64_t orig_entry, uint32_t bits)
> +{
> + uint64_t new_entry = orig_entry | bits;
> +
> + /* Write the updated bottom 32-bits */
> + if (qemu_tcg_mttcg_enabled()) {
> + uint32_t old_le = cpu_to_le32(orig_entry);
> + uint32_t new_le = cpu_to_le32(new_entry);
> + MemTxResult result;
> + uint32_t old_ret;
> +
> + old_ret = address_space_cmpxchgl_notdirty(cs->as, addr,
> + old_le, new_le,
> + MEMTXATTRS_UNSPECIFIED,
> + &result);
> + if (result == MEMTX_OK) {
> + if (old_ret != old_le && old_ret != new_le) {
> + new_entry = 0;
> + }
> + return new_entry;
> + }
> +
> + /* Do we need to support this case where PTEs aren't in RAM ?
> + *
> + * For now fallback to non-atomic case
> + */
> + }
> +
> + x86_stl_phys_notdirty(cs, addr, new_entry);
> +
> + return new_entry;
> +}
> +
> static hwaddr get_hphys(CPUState *cs, hwaddr gphys, MMUAccessType
> access_type,
> int *prot)
> {
> CPUX86State *env = &X86_CPU(cs)->env;
> - uint64_t rsvd_mask = PG_HI_RSVD_MASK;
> + uint64_t rsvd_mask;
> uint64_t ptep, pte;
> uint64_t exit_info_1 = 0;
> target_ulong pde_addr, pte_addr;
> @@ -172,6 +206,8 @@ static hwaddr get_hphys(CPUState *cs, hwaddr gphys,
> MMUAccessType access_type,
> return gphys;
> }
>
> + restart:
> + rsvd_mask = PG_HI_RSVD_MASK;
> if (!(env->nested_pg_mode & SVM_NPT_NXE)) {
> rsvd_mask |= PG_NX_MASK;
> }
> @@ -198,8 +234,10 @@ static hwaddr get_hphys(CPUState *cs, hwaddr gphys,
> MMUAccessType access_type,
> goto do_fault_rsvd;
> }
> if (!(pml4e & PG_ACCESSED_MASK)) {
> - pml4e |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pml4e_addr, pml4e);
> + pml4e = update_entry(cs, pml4e_addr, pml4e,
> PG_ACCESSED_MASK);
> + if (!pml4e) {
> + goto restart;
> + }
> }
> ptep &= pml4e ^ PG_NX_MASK;
> pdpe_addr = (pml4e & PG_ADDRESS_MASK) +
> @@ -213,8 +251,10 @@ static hwaddr get_hphys(CPUState *cs, hwaddr gphys,
> MMUAccessType access_type,
> }
> ptep &= pdpe ^ PG_NX_MASK;
> if (!(pdpe & PG_ACCESSED_MASK)) {
> - pdpe |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pdpe_addr, pdpe);
> + pdpe = update_entry(cs, pdpe_addr, pdpe, PG_ACCESSED_MASK);
> + if (!pdpe) {
> + goto restart;
> + }
> }
> if (pdpe & PG_PSE_MASK) {
> /* 1 GB page */
> @@ -256,8 +296,10 @@ static hwaddr get_hphys(CPUState *cs, hwaddr gphys,
> MMUAccessType access_type,
> }
> /* 4 KB page */
> if (!(pde & PG_ACCESSED_MASK)) {
> - pde |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pde_addr, pde);
> + pde = update_entry(cs, pde_addr, pde, PG_ACCESSED_MASK);
> + if (!pde) {
> + goto restart;
> + }
> }
> pte_addr = (pde & PG_ADDRESS_MASK) + (((gphys >> 12) & 0x1ff) << 3);
> pte = x86_ldq_phys(cs, pte_addr);
> @@ -295,8 +337,10 @@ static hwaddr get_hphys(CPUState *cs, hwaddr gphys,
> MMUAccessType access_type,
> }
>
> if (!(pde & PG_ACCESSED_MASK)) {
> - pde |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pde_addr, pde);
> + pde = update_entry(cs, pde_addr, pde, PG_ACCESSED_MASK);
> + if (!pde) {
> + goto restart;
> + }
> }
>
> /* page directory entry */
> @@ -376,7 +420,7 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
> int error_code = 0;
> int is_dirty, prot, page_size, is_write, is_user;
> hwaddr paddr;
> - uint64_t rsvd_mask = PG_HI_RSVD_MASK;
> + uint64_t rsvd_mask;
> uint32_t page_offset;
> target_ulong vaddr;
>
> @@ -401,6 +445,8 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
> goto do_mapping;
> }
>
> + restart:
> + rsvd_mask = PG_HI_RSVD_MASK;
> if (!(env->efer & MSR_EFER_NXE)) {
> rsvd_mask |= PG_NX_MASK;
> }
> @@ -436,8 +482,10 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
> goto do_fault_rsvd;
> }
> if (!(pml5e & PG_ACCESSED_MASK)) {
> - pml5e |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pml5e_addr, pml5e);
> + pml5e = update_entry(cs, pml5e_addr, pml5e,
> PG_ACCESSED_MASK);
> + if (!pml5e) {
> + goto restart;
> + }
> }
> ptep = pml5e ^ PG_NX_MASK;
> } else {
> @@ -456,8 +504,10 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
> goto do_fault_rsvd;
> }
> if (!(pml4e & PG_ACCESSED_MASK)) {
> - pml4e |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pml4e_addr, pml4e);
> + pml4e = update_entry(cs, pml4e_addr, pml4e,
> PG_ACCESSED_MASK);
> + if (!pml4e) {
> + goto restart;
> + }
> }
> ptep &= pml4e ^ PG_NX_MASK;
> pdpe_addr = ((pml4e & PG_ADDRESS_MASK) + (((addr >> 30) & 0x1ff)
> << 3)) &
> @@ -472,8 +522,10 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
> }
> ptep &= pdpe ^ PG_NX_MASK;
> if (!(pdpe & PG_ACCESSED_MASK)) {
> - pdpe |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pdpe_addr, pdpe);
> + pdpe = update_entry(cs, pdpe_addr, pdpe, PG_ACCESSED_MASK);
> + if (!pdpe) {
> + goto restart;
> + }
> }
> if (pdpe & PG_PSE_MASK) {
> /* 1 GB page */
> @@ -520,8 +572,10 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
> }
> /* 4 KB page */
> if (!(pde & PG_ACCESSED_MASK)) {
> - pde |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pde_addr, pde);
> + pde = update_entry(cs, pde_addr, pde, PG_ACCESSED_MASK);
> + if (!pde) {
> + goto restart;
> + }
> }
> pte_addr = ((pde & PG_ADDRESS_MASK) + (((addr >> 12) & 0x1ff) << 3))
> &
> a20_mask;
> @@ -563,8 +617,10 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
> int size,
> }
>
> if (!(pde & PG_ACCESSED_MASK)) {
> - pde |= PG_ACCESSED_MASK;
> - x86_stl_phys_notdirty(cs, pde_addr, pde);
> + pde = update_entry(cs, pde_addr, pde, PG_ACCESSED_MASK);
> + if (!pde) {
> + goto restart;
> + }
> }
>
> /* page directory entry */
> @@ -634,11 +690,11 @@ do_check_protect_pse36:
> /* yes, it can! */
> is_dirty = is_write && !(pte & PG_DIRTY_MASK);
> if (!(pte & PG_ACCESSED_MASK) || is_dirty) {
> - pte |= PG_ACCESSED_MASK;
> - if (is_dirty) {
> - pte |= PG_DIRTY_MASK;
> + pte = update_entry(cs, pte_addr, pte,
> + PG_ACCESSED_MASK | (is_dirty ? PG_DIRTY_MASK :
> 0));
> + if (!pte) {
> + goto restart;
> }
> - x86_stl_phys_notdirty(cs, pte_addr, pte);
> }
>
> if (!(pte & PG_DIRTY_MASK)) {