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Message-ID: <00f392ee-8165-cc53-911c-732110470a16@amd.com>
Date: Fri, 25 Jun 2021 08:28:26 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, Peter Gonda <pgonda@...gle.com>,
Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH 1/2] Revert "KVM: x86: Truncate reported guest MAXPHYADDR
to C-bit if SEV is supported"
On 6/24/21 9:03 PM, Sean Christopherson wrote:
> It turns out that non-SEV guest can indeed use bit 47, and the unexpected
> reserved #PF observed when setting bit 47 is due to a magic 12gb
> HyperTransport region being off limits, even for GPAs. Per Tom:
>
> I think you may be hitting a special HT region that is at the top 12GB
> of the 48-bit memory range and is reserved, even for GPAs. Can you
> somehow get the test to use an address below 0xfffd_0000_0000? That
> would show that bit 47 is valid for the legacy guest while staying out
> of the HT region.
>
> And indeed, accessing 0xfffd00000000 generates a reserved #PF, while
> dropping down a single page to 0xfffcfffff000 does not.
>
> This reverts commit 3675f005c87c4026713c9f863924de511fdd36c4.
>
> Cc: Peter Gonda <pgonda@...gle.com>
> Cc: Brijesh Singh <brijesh.singh@....com>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Acked-by: Tom Lendacky <thomas.lendacky@....com>
> ---
> arch/x86/kvm/cpuid.c | 11 -----------
> arch/x86/kvm/svm/svm.c | 37 ++++++++-----------------------------
> arch/x86/kvm/x86.c | 3 ---
> arch/x86/kvm/x86.h | 1 -
> 4 files changed, 8 insertions(+), 44 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0edda1fc4fe7..ca7866d63e98 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -955,17 +955,6 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
> else if (!g_phys_as)
> g_phys_as = phys_as;
>
> - /*
> - * The exception to the exception is if hardware supports SEV,
> - * in which case the C-bit is reserved for non-SEV guests and
> - * isn't a GPA bit for SEV guests.
> - *
> - * Note, KVM always reports '0' for the number of reduced PA
> - * bits (see 0x8000001F).
> - */
> - if (tdp_enabled && sev_c_bit)
> - g_phys_as = min(g_phys_as, (unsigned int)sev_c_bit);
> -
> entry->eax = g_phys_as | (virt_as << 8);
> entry->edx = 0;
> cpuid_entry_override(entry, CPUID_8000_0008_EBX);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 616b9679ddcc..8834822c00cd 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -860,26 +860,6 @@ static void shrink_ple_window(struct kvm_vcpu *vcpu)
> }
> }
>
> -static __init u8 svm_get_c_bit(bool sev_only)
> -{
> - unsigned int eax, ebx, ecx, edx;
> - u64 msr;
> -
> - if (cpuid_eax(0x80000000) < 0x8000001f)
> - return 0;
> -
> - if (rdmsrl_safe(MSR_AMD64_SYSCFG, &msr) ||
> - !(msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT))
> - return 0;
> -
> - cpuid(0x8000001f, &eax, &ebx, &ecx, &edx);
> -
> - if (sev_only && !(eax & feature_bit(SEV)))
> - return 0;
> -
> - return ebx & 0x3f;
> -}
> -
> /*
> * The default MMIO mask is a single bit (excluding the present bit),
> * which could conflict with the memory encryption bit. Check for
> @@ -889,13 +869,18 @@ static __init u8 svm_get_c_bit(bool sev_only)
> static __init void svm_adjust_mmio_mask(void)
> {
> unsigned int enc_bit, mask_bit;
> - u64 mask;
> + u64 msr, mask;
> +
> + /* If there is no memory encryption support, use existing mask */
> + if (cpuid_eax(0x80000000) < 0x8000001f)
> + return;
>
> /* If memory encryption is not enabled, use existing mask */
> - enc_bit = svm_get_c_bit(false);
> - if (!enc_bit)
> + rdmsrl(MSR_AMD64_SYSCFG, msr);
> + if (!(msr & MSR_AMD64_SYSCFG_MEM_ENCRYPT))
> return;
>
> + enc_bit = cpuid_ebx(0x8000001f) & 0x3f;
> mask_bit = boot_cpu_data.x86_phys_bits;
>
> /* Increment the mask bit if it is the same as the encryption bit */
> @@ -1028,12 +1013,6 @@ static __init int svm_hardware_setup(void)
> kvm_configure_mmu(npt_enabled, get_max_npt_level(), PG_LEVEL_1G);
> pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
>
> - /*
> - * The SEV C-bit location is needed to correctly enumeration guest
> - * MAXPHYADDR even if SEV is not fully supported.
> - */
> - sev_c_bit = svm_get_c_bit(true);
> -
> /* Note, SEV setup consumes npt_enabled. */
> sev_hardware_setup();
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4a597aafe637..13905ef5bb48 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -209,9 +209,6 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
> | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> | XFEATURE_MASK_PKRU)
>
> -u8 __read_mostly sev_c_bit;
> -EXPORT_SYMBOL_GPL(sev_c_bit);
> -
> u64 __read_mostly host_efer;
> EXPORT_SYMBOL_GPL(host_efer);
>
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index bc3f5c9e3708..44ae10312740 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -328,7 +328,6 @@ extern u64 host_xcr0;
> extern u64 supported_xcr0;
> extern u64 host_xss;
> extern u64 supported_xss;
> -extern u8 sev_c_bit;
>
> static inline bool kvm_mpx_supported(void)
> {
>
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