Re: [PATCH v9 04/12] x86/mm: get INVLPGB count max from CPUID

[Vern Hao <haoxing990@gmail.com>]

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On 2025/2/11 00:48, Rik van Riel wrote:
> On Mon, 2025-02-10 at 15:30 +0800, Vern Hao wrote:
>> I do some test on my  Machine with AMD EPYC 7K83,  these patches work
>> on my host,  but failed on my guest with qemu.
>>
>> in host, use lscpu cmd, you can see  invlpgb in flags,  but in guest
>> no.
>>
>> So are you plan to support it in guest?
> How exactly did it fail in the guest?
>
> Did the guest simply not use INVLPGB because that
> CPUID flag was not presented in the CPUID that
> qemu shows to the guest, or did things crash somehow?
i support these patches in host and guest, and add this patch to support 
cpuid flags in kvm.

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index db3838667466..fd21d9438137 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -488,7 +488,7 @@ static inline int __do_cpuid_func(struct 
kvm_cpuid_entry2 *entry, u32 function,

         /* cpuid 0x80000008.ebx */
         const u32 kvm_cpuid_8000_0008_ebx_x86_features =
-               F(CLZERO) | F(XSAVEERPTR) |
+              F(CLZERO) | F(XSAVEERPTR) | F(INVLPGB) |
                 F(WBNOINVD) | F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | 
F(VIRT_SSBD) |
                 F(AMD_SSB_NO) | F(AMD_STIBP) | F(AMD_STIBP_ALWAYS_ON);

But in guest,  use lscpu cmd, i still can not see  invlpgb,  so  i just 
wonder where is wrong ?

>
> My understanding is that while INVLPGB can work
> in guests, actually implementing that is a whole
> other can of worms, and definitely not something
> we should try to tackle at the same time as bare
> metal support.
>
> A TLB flush hypercall, with IRQ-less flushing on
> the hypervisor side will probably get us 90% of
> the way there, potentially with less overall
> complexity than actually supporting INVLPGB in
> the guest.
>

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