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

Nadav Amit
2022-10-03T17:36:10Z

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);
--
2.34.1