[PATCH v3 0/3] kvm: add support for guest physical bits

[PATCH v3 0/3] kvm: add support for guest physical bits

[Gerd Hoffmann]

The matching kernel bits are here:
https://lore.kernel.org/kvm/20240313125844.912415-1-kraxel@redhat.com/T/

ovmf test patches are here:
https://github.com/kraxel/edk2/commits/devel/guest-phys-bits/

Gerd Hoffmann (3):
  [debug] log kvm supported cpuid
  kvm: add support for guest physical bits
  target/i386: add guest-phys-bits cpu property

 target/i386/cpu.h         |  1 +
 target/i386/cpu.c         | 14 ++++++++++++++
 target/i386/kvm/kvm-cpu.c | 32 +++++++++++++++++++++++++++++++-
 target/i386/kvm/kvm.c     | 14 ++++++++++++++
 4 files changed, 60 insertions(+), 1 deletion(-)

-- 
2.44.0

[PATCH v3 1/3] [debug] log kvm supported cpuid

[Gerd Hoffmann]

---
 target/i386/kvm/kvm.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index e68cbe929302..2f5e3b9febf9 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -276,6 +276,20 @@ static struct kvm_cpuid2 *try_get_cpuid(KVMState *s, int max)
             exit(1);
         }
     }
+
+    {
+        int i;
+
+        for (i = 0; i < cpuid->nent; i++) {
+            fprintf(stderr, "cpuid: %8x/%d - %8x %8x %8x %8x\n",
+                    cpuid->entries[i].function,
+                    cpuid->entries[i].index,
+                    cpuid->entries[i].eax,
+                    cpuid->entries[i].ebx,
+                    cpuid->entries[i].ecx,
+                    cpuid->entries[i].edx);
+        }
+    }
     return cpuid;
 }
 
-- 
2.44.0

[PATCH v3 2/3] kvm: add support for guest physical bits

[Gerd Hoffmann]

Query kvm for supported guest physical address bits, in cpuid
function 80000008, eax[23:16].  Usually this is identical to host
physical address bits.  With NPT or EPT being used this might be
restricted to 48 (max 4-level paging address space size) even if
the host cpu supports more physical address bits.

When set pass this to the guest, using cpuid too.  Guest firmware
can use this to figure how big the usable guest physical address
space is, so PCI bar mapping are actually reachable.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 target/i386/cpu.h         |  1 +
 target/i386/cpu.c         |  1 +
 target/i386/kvm/kvm-cpu.c | 32 +++++++++++++++++++++++++++++++-
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 952174bb6f52..d427218827f6 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -2026,6 +2026,7 @@ struct ArchCPU {
 
     /* Number of physical address bits supported */
     uint32_t phys_bits;
+    uint32_t guest_phys_bits;
 
     /* in order to simplify APIC support, we leave this pointer to the
        user */
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 9a210d8d9290..c88c895a5b3e 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6570,6 +6570,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
         if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
             /* 64 bit processor */
              *eax |= (cpu_x86_virtual_addr_width(env) << 8);
+             *eax |= (cpu->guest_phys_bits << 16);
         }
         *ebx = env->features[FEAT_8000_0008_EBX];
         if (cs->nr_cores * cs->nr_threads > 1) {
diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
index 9c791b7b0520..a2b7bfaeadf8 100644
--- a/target/i386/kvm/kvm-cpu.c
+++ b/target/i386/kvm/kvm-cpu.c
@@ -18,10 +18,36 @@
 #include "kvm_i386.h"
 #include "hw/core/accel-cpu.h"
 
+static void kvm_set_guest_phys_bits(CPUState *cs)
+{
+    X86CPU *cpu = X86_CPU(cs);
+    uint32_t eax, guest_phys_bits;
+
+    if (!cpu->host_phys_bits) {
+        return;
+    }
+
+    eax = kvm_arch_get_supported_cpuid(cs->kvm_state, 0x80000008, 0, R_EAX);
+    guest_phys_bits = (eax >> 16) & 0xff;
+    if (!guest_phys_bits) {
+        return;
+    }
+
+    if (cpu->guest_phys_bits == 0 ||
+        cpu->guest_phys_bits > guest_phys_bits) {
+        cpu->guest_phys_bits = guest_phys_bits;
+    }
+
+    if (cpu->guest_phys_bits > cpu->host_phys_bits_limit) {
+        cpu->guest_phys_bits = cpu->host_phys_bits_limit;
+    }
+}
+
 static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
 {
     X86CPU *cpu = X86_CPU(cs);
     CPUX86State *env = &cpu->env;
+    bool ret;
 
     /*
      * The realize order is important, since x86_cpu_realize() checks if
@@ -50,7 +76,11 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
                                                    MSR_IA32_UCODE_REV);
         }
     }
-    return host_cpu_realizefn(cs, errp);
+    ret = host_cpu_realizefn(cs, errp);
+
+    kvm_set_guest_phys_bits(cs);
+
+    return ret;
 }
 
 static bool lmce_supported(void)
-- 
2.44.0

[PATCH v3 3/3] target/i386: add guest-phys-bits cpu property

[Gerd Hoffmann]

Allows to set guest-phys-bits (cpuid leaf 80000008, eax[23:16])
via -cpu $model,guest-phys-bits=$nr.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 target/i386/cpu.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index c88c895a5b3e..e0d73b6ec654 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -7380,6 +7380,14 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
         if (cpu->phys_bits == 0) {
             cpu->phys_bits = TCG_PHYS_ADDR_BITS;
         }
+        if (cpu->guest_phys_bits &&
+            (cpu->guest_phys_bits > cpu->phys_bits ||
+            cpu->guest_phys_bits < 32)) {
+            error_setg(errp, "guest-phys-bits should be between 32 and %u "
+                             " (but is %u)",
+                             cpu->phys_bits, cpu->guest_phys_bits);
+            return;
+        }
     } else {
         /* For 32 bit systems don't use the user set value, but keep
          * phys_bits consistent with what we tell the guest.
@@ -7388,6 +7396,10 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
             error_setg(errp, "phys-bits is not user-configurable in 32 bit");
             return;
         }
+        if (cpu->guest_phys_bits != 0) {
+            error_setg(errp, "guest-phys-bits is not user-configurable in 32 bit");
+            return;
+        }
 
         if (env->features[FEAT_1_EDX] & (CPUID_PSE36 | CPUID_PAE)) {
             cpu->phys_bits = 36;
@@ -7888,6 +7900,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_BOOL("x-force-features", X86CPU, force_features, false),
     DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
     DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 0),
+    DEFINE_PROP_UINT32("guest-phys-bits", X86CPU, guest_phys_bits, 0),
     DEFINE_PROP_BOOL("host-phys-bits", X86CPU, host_phys_bits, false),
     DEFINE_PROP_UINT8("host-phys-bits-limit", X86CPU, host_phys_bits_limit, 0),
     DEFINE_PROP_BOOL("fill-mtrr-mask", X86CPU, fill_mtrr_mask, true),
-- 
2.44.0

Re: [PATCH v3 2/3] kvm: add support for guest physical bits

[Tao Su]

On Wed, Mar 13, 2024 at 02:27:18PM +0100, Gerd Hoffmann wrote:
> Query kvm for supported guest physical address bits, in cpuid
> function 80000008, eax[23:16].  Usually this is identical to host
> physical address bits.  With NPT or EPT being used this might be
> restricted to 48 (max 4-level paging address space size) even if
> the host cpu supports more physical address bits.
> 
> When set pass this to the guest, using cpuid too.  Guest firmware
> can use this to figure how big the usable guest physical address
> space is, so PCI bar mapping are actually reachable.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  target/i386/cpu.h         |  1 +
>  target/i386/cpu.c         |  1 +
>  target/i386/kvm/kvm-cpu.c | 32 +++++++++++++++++++++++++++++++-
>  3 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 952174bb6f52..d427218827f6 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -2026,6 +2026,7 @@ struct ArchCPU {
>  
>      /* Number of physical address bits supported */
>      uint32_t phys_bits;
> +    uint32_t guest_phys_bits;
>  
>      /* in order to simplify APIC support, we leave this pointer to the
>         user */
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 9a210d8d9290..c88c895a5b3e 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6570,6 +6570,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
>          if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
>              /* 64 bit processor */
>               *eax |= (cpu_x86_virtual_addr_width(env) << 8);
> +             *eax |= (cpu->guest_phys_bits << 16);
>          }
>          *ebx = env->features[FEAT_8000_0008_EBX];
>          if (cs->nr_cores * cs->nr_threads > 1) {
> diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
> index 9c791b7b0520..a2b7bfaeadf8 100644
> --- a/target/i386/kvm/kvm-cpu.c
> +++ b/target/i386/kvm/kvm-cpu.c
> @@ -18,10 +18,36 @@
>  #include "kvm_i386.h"
>  #include "hw/core/accel-cpu.h"
>  
> +static void kvm_set_guest_phys_bits(CPUState *cs)
> +{
> +    X86CPU *cpu = X86_CPU(cs);
> +    uint32_t eax, guest_phys_bits;
> +
> +    if (!cpu->host_phys_bits) {
> +        return;
> +    }
> +
> +    eax = kvm_arch_get_supported_cpuid(cs->kvm_state, 0x80000008, 0, R_EAX);
> +    guest_phys_bits = (eax >> 16) & 0xff;
> +    if (!guest_phys_bits) {
> +        return;
> +    }
> +
> +    if (cpu->guest_phys_bits == 0 ||
> +        cpu->guest_phys_bits > guest_phys_bits) {
> +        cpu->guest_phys_bits = guest_phys_bits;
> +    }
> +
> +    if (cpu->guest_phys_bits > cpu->host_phys_bits_limit) {
> +        cpu->guest_phys_bits = cpu->host_phys_bits_limit;

host_phys_bits_limit is zero by default, so I think it is better to be
like:

        if (cpu->host_phys_bits_limit &&
            cpu->guest_phys_bits > cpu->host_phys_bits_limit) {
            cpu->guest_phys_bits = cpu->host_phys_bits_limit;
        }

> +    }
> +}
> +
>  static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
>  {
>      X86CPU *cpu = X86_CPU(cs);
>      CPUX86State *env = &cpu->env;
> +    bool ret;
>  
>      /*
>       * The realize order is important, since x86_cpu_realize() checks if
> @@ -50,7 +76,11 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
>                                                     MSR_IA32_UCODE_REV);
>          }
>      }
> -    return host_cpu_realizefn(cs, errp);
> +    ret = host_cpu_realizefn(cs, errp);
> +
> +    kvm_set_guest_phys_bits(cs);
> +
> +    return ret;
>  }
>  
>  static bool lmce_supported(void)
> -- 
> 2.44.0
> 
> 

Re: [PATCH v3 2/3] kvm: add support for guest physical bits

[Xiaoyao Li]

On 3/13/2024 9:27 PM, Gerd Hoffmann wrote:
> Query kvm for supported guest physical address bits, in cpuid
> function 80000008, eax[23:16].  Usually this is identical to host
> physical address bits.  With NPT or EPT being used this might be
> restricted to 48 (max 4-level paging address space size) even if
> the host cpu supports more physical address bits.
> 
> When set pass this to the guest, using cpuid too.  Guest firmware
> can use this to figure how big the usable guest physical address
> space is, so PCI bar mapping are actually reachable.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>   target/i386/cpu.h         |  1 +
>   target/i386/cpu.c         |  1 +
>   target/i386/kvm/kvm-cpu.c | 32 +++++++++++++++++++++++++++++++-
>   3 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 952174bb6f52..d427218827f6 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -2026,6 +2026,7 @@ struct ArchCPU {
>   
>       /* Number of physical address bits supported */
>       uint32_t phys_bits;
> +    uint32_t guest_phys_bits;
>   
>       /* in order to simplify APIC support, we leave this pointer to the
>          user */
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 9a210d8d9290..c88c895a5b3e 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6570,6 +6570,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
>           if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
>               /* 64 bit processor */
>                *eax |= (cpu_x86_virtual_addr_width(env) << 8);
> +             *eax |= (cpu->guest_phys_bits << 16);
>           }
>           *ebx = env->features[FEAT_8000_0008_EBX];
>           if (cs->nr_cores * cs->nr_threads > 1) {
> diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
> index 9c791b7b0520..a2b7bfaeadf8 100644
> --- a/target/i386/kvm/kvm-cpu.c
> +++ b/target/i386/kvm/kvm-cpu.c
> @@ -18,10 +18,36 @@
>   #include "kvm_i386.h"
>   #include "hw/core/accel-cpu.h"
>   
> +static void kvm_set_guest_phys_bits(CPUState *cs)
> +{
> +    X86CPU *cpu = X86_CPU(cs);
> +    uint32_t eax, guest_phys_bits;
> +
> +    if (!cpu->host_phys_bits) {
> +        return;
> +    }

This needs explanation of why. What if users set the phys-bits to 
exactly host's value, via "-cpu xxx,phys-bits=host's value"?


> +    eax = kvm_arch_get_supported_cpuid(cs->kvm_state, 0x80000008, 0, R_EAX);
> +    guest_phys_bits = (eax >> 16) & 0xff;
> +    if (!guest_phys_bits) {
> +        return;
> +    }
> +
> +    if (cpu->guest_phys_bits == 0 ||
> +        cpu->guest_phys_bits > guest_phys_bits) {
> +        cpu->guest_phys_bits = guest_phys_bits;
> +    }
> +
> +    if (cpu->guest_phys_bits > cpu->host_phys_bits_limit) {
> +        cpu->guest_phys_bits = cpu->host_phys_bits_limit;
> +    }
> +}
> +
>   static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
>   {
>       X86CPU *cpu = X86_CPU(cs);
>       CPUX86State *env = &cpu->env;
> +    bool ret;
>   
>       /*
>        * The realize order is important, since x86_cpu_realize() checks if
> @@ -50,7 +76,11 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
>                                                      MSR_IA32_UCODE_REV);
>           }
>       }
> -    return host_cpu_realizefn(cs, errp);
> +    ret = host_cpu_realizefn(cs, errp);

We need to check ret and return if !ret;

> +    kvm_set_guest_phys_bits(cs);
> +
> +    return ret;
>   }
>   
>   static bool lmce_supported(void)

Re: [PATCH v3 2/3] kvm: add support for guest physical bits

[Gerd Hoffmann]

  Hi,

> > +    if (cpu->guest_phys_bits > cpu->host_phys_bits_limit) {
> > +        cpu->guest_phys_bits = cpu->host_phys_bits_limit;
> 
> host_phys_bits_limit is zero by default, so I think it is better to be
> like:
> 
>         if (cpu->host_phys_bits_limit &&
>             cpu->guest_phys_bits > cpu->host_phys_bits_limit) {
>             cpu->guest_phys_bits = cpu->host_phys_bits_limit;
>         }

Good point, fixed.

thanks,
  Gerd

Re: [PATCH v3 2/3] kvm: add support for guest physical bits

[Gerd Hoffmann]

  Hi,

> > diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
> > index 9c791b7b0520..a2b7bfaeadf8 100644
> > --- a/target/i386/kvm/kvm-cpu.c
> > +++ b/target/i386/kvm/kvm-cpu.c
> > @@ -18,10 +18,36 @@
> >   #include "kvm_i386.h"
> >   #include "hw/core/accel-cpu.h"
> > +static void kvm_set_guest_phys_bits(CPUState *cs)
> > +{
> > +    X86CPU *cpu = X86_CPU(cs);
> > +    uint32_t eax, guest_phys_bits;
> > +
> > +    if (!cpu->host_phys_bits) {
> > +        return;
> > +    }
> 
> This needs explanation of why. What if users set the phys-bits to exactly
> host's value, via "-cpu xxx,phys-bits=host's value"?

If host_phys_bits is not enabled it is possible to set phys-bits to any
value today (including invalid values not supported by the host).  With
this the same applies to guest_phys_bits.

My intention was to continue allowing any guest_phys_bits + phys_bits
with TCG, for testing purposes.  But thinking again this logic is
flawed, if TCG is used the control flow doesn't land here in the first
place.

So, I think this can be dropped.

> > +    ret = host_cpu_realizefn(cs, errp);
> 
> We need to check ret and return if !ret;

Fixed.

thanks,
  Gerd