Re: [PATCH v2] x86/PCI: Prefer MMIO over PIO on all hypervisor

["H. Peter Anvin" <hpa@zytor.com>]

On October 3, 2022 2:28:40 PM PDT, Nadav Amit <namit@vmware.com> wrote:
>On Oct 3, 2022, at 2:06 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>
>> ⚠ External Email
>> 
>> On October 3, 2022 10:34:15 AM PDT, Nadav Amit <namit@vmware.com> wrote:
>>> On Oct 3, 2022, at 8:03 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>> 
>>>> Not my but rather PCI maintainer's call but IMHO dropping 'const' is
>>>> better, introducing a new global var is our 'last resort' and should be
>>>> avoided whenever possible. Alternatively, you can add a
>>>> raw_pci_ext_ops_preferred() function checking somethin within 'struct
>>>> hypervisor_x86' but I'm unsure if it's better.
>>>> 
>>>> Also, please check Alex' question/suggestion.
>>> 
>>> Here is my take (and Ajay knows probably more than me):
>>> 
>>> Looking briefly on MCFG, I do not see a clean way of using the ACPI table.
>>> The two options are either to use a reserved field (which who knows, might
>>> be used one day) or some OEM ID. I am also not familiar with
>>> PCI_COMMAND.MEMORY=0, so Ajay can hopefully give some answer about that.
>>> 
>>> Anyhow, I understand (although not relate) to the objection for a new global
>>> variable. How about explicitly calling this hardware bug a “bug” and using
>>> the proper infrastructure? Calling it explicitly a bug may even push whoever
>>> can to resolve it.
>>> 
>>> IOW, how about doing something along the lines of (not tested):
>>> 
>>> 
>>> -- >8 --
>>> 
>>> Subject: [PATCH] x86/PCI: Prefer MMIO over PIO on VMware hypervisor
>>> 
>>> ---
>>> arch/x86/include/asm/cpufeatures.h | 1 +
>>> arch/x86/kernel/cpu/common.c       | 2 ++
>>> arch/x86/kernel/cpu/vmware.c       | 2 ++
>>> arch/x86/pci/common.c              | 6 ++++--
>>> 4 files changed, 9 insertions(+), 2 deletions(-)
>>> 
>>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>>> index ef4775c6db01..216b6f357b6d 100644
>>> --- a/arch/x86/include/asm/cpufeatures.h
>>> +++ b/arch/x86/include/asm/cpufeatures.h
>>> @@ -460,5 +460,6 @@
>>> #define X86_BUG_MMIO_UNKNOWN          X86_BUG(26) /* CPU is too old and its MMIO Stale Data status is unknown */
>>> #define X86_BUG_RETBLEED              X86_BUG(27) /* CPU is affected by RETBleed */
>>> #define X86_BUG_EIBRS_PBRSB           X86_BUG(28) /* EIBRS is vulnerable to Post Barrier RSB Predictions */
>>> +#define X86_BUG_ECAM_MMIO             X86_BUG(29) /* ECAM MMIO is buggy and PIO is preferable */
>>> 
>>> #endif /* _ASM_X86_CPUFEATURES_H */
>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>>> index 3e508f239098..c94175fa304b 100644
>>> --- a/arch/x86/kernel/cpu/common.c
>>> +++ b/arch/x86/kernel/cpu/common.c
>>> @@ -1299,6 +1299,8 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
>>> {
>>>      u64 ia32_cap = x86_read_arch_cap_msr();
>>> 
>>> +      setup_force_cpu_bug(X86_BUG_ECAM_MMIO);
>>> +
>>>      /* Set ITLB_MULTIHIT bug if cpu is not in the whitelist and not mitigated */
>>>      if (!cpu_matches(cpu_vuln_whitelist, NO_ITLB_MULTIHIT) &&
>>>          !(ia32_cap & ARCH_CAP_PSCHANGE_MC_NO))
>>> diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
>>> index 02039ec3597d..8903776284a6 100644
>>> --- a/arch/x86/kernel/cpu/vmware.c
>>> +++ b/arch/x86/kernel/cpu/vmware.c
>>> @@ -385,6 +385,8 @@ static void __init vmware_set_capabilities(void)
>>>              setup_force_cpu_cap(X86_FEATURE_VMCALL);
>>>      else if (vmware_hypercall_mode == CPUID_VMWARE_FEATURES_ECX_VMMCALL)
>>>              setup_force_cpu_cap(X86_FEATURE_VMW_VMMCALL);
>>> +
>>> +      setup_clear_cpu_cap(X86_BUG_ECAM_MMIO);
>>> }
>>> 
>>> static void __init vmware_platform_setup(void)
>>> diff --git a/arch/x86/pci/common.c b/arch/x86/pci/common.c
>>> index ddb798603201..bc81cf4c1014 100644
>>> --- a/arch/x86/pci/common.c
>>> +++ b/arch/x86/pci/common.c
>>> @@ -40,7 +40,8 @@ const struct pci_raw_ops *__read_mostly raw_pci_ext_ops;
>>> int raw_pci_read(unsigned int domain, unsigned int bus, unsigned int devfn,
>>>                                              int reg, int len, u32 *val)
>>> {
>>> -      if (domain == 0 && reg < 256 && raw_pci_ops)
>>> +      if (domain == 0 && reg < 256 && raw_pci_ops &&
>>> +          (boot_cpu_has_bug(X86_BUG_ECAM_MMIO) || !raw_pci_ext_ops))
>>>              return raw_pci_ops->read(domain, bus, devfn, reg, len, val);
>>>      if (raw_pci_ext_ops)
>>>              return raw_pci_ext_ops->read(domain, bus, devfn, reg, len, val);
>>> @@ -50,7 +51,8 @@ int raw_pci_read(unsigned int domain, unsigned int bus, unsigned int devfn,
>>> int raw_pci_write(unsigned int domain, unsigned int bus, unsigned int devfn,
>>>                                              int reg, int len, u32 val)
>>> {
>>> -      if (domain == 0 && reg < 256 && raw_pci_ops)
>>> +      if (domain == 0 && reg < 256 && raw_pci_ops &&
>>> +          (boot_cpu_has_bug(X86_BUG_ECAM_MMIO) || !raw_pci_ext_ops))
>>>              return raw_pci_ops->write(domain, bus, devfn, reg, len, val);
>>>      if (raw_pci_ext_ops)
>>>              return raw_pci_ext_ops->write(domain, bus, devfn, reg, len, val);
>> 
>> Also... any reason we can't just set raw_pci_ops == raw_ext_pci_ops for the case when the latter is preferred, and dispense with the conditionals in the use path? Similarly, raw_ext_pci_ops could be pointed to error routines instead of left at NULL.
>
>I understood from Ajay that the initialization of raw_ext_pci_ops can be
>done after the hypervisor initialization takes place, so doing what exactly
>what you proposed by is not possible. It can probably be resolved, but I do
>not think the end result would be simpler or cleaner. I think that the “bug”
>solution really conveys the behavior.
>
>IIUC performance would not be noticeable affected by 2 more conditional
>branches.
>
>

Isn't that exactly what you would want?!?